src: silence compiler warnings 1

Most of the changes aim to silence warnings by adding casts.

An assortment of other issues, mainly compiler warnings, resolved:

- unreachable code fixed by using `goto` in
  `publickey_response_success()` in `publickey.c`.

- potentially uninitialized variable in `sftp_open()`.

- MSVS-specific bogus warnings with `nid_type` in `kex.c`.

- check result of `kex_session_ecdh_curve_type()`.

- add missing function declarations.

- type changes to fit values without casts:
  - `cmd_len` in `scp_recv()` and `scp_send()`: `int` -> `size_t`
  - `Blowfish_expandstate()`, `Blowfish_expand0state()` loop counters:
    `uint16_t` -> `int`
  - `RECV_SEND_ALL()`: `int` -> `ssize_t`
  - `shell_quotearg()` -> `unsigned` -> `size_t`
  - `sig_len` in `_libssh2_mbedtls_rsa_sha2_sign()`:
    `unsigned` -> `size_t`
  - `prefs_len` in `libssh2_session_method_pref()`: `int` -> `size_t`
  - `firstsec` in `_libssh2_debug_low()`: `int` -> `long`
  - `method_len` in `libssh2_session_method_pref()`: `int` -> `size_t`

- simplify `_libssh2_ntohu64()`.

- fix `LIBSSH2_INT64_T_FORMAT` for MinGW.

- fix gcc warning by not using a bit field for
  `burn_optimistic_kexinit`.

- fix unused variable warning in `_libssh2_cipher_crypt()` in
  `libgcrypt.c`.

- fix unused variables with `HAVE_DISABLED_NONBLOCKING`.

- avoid const stripping with `BIO_new_mem_buf()` and OpenSSL 1.0.2 and
  newer.

- add a missing const in `wincng.h`.

- FIXME added for public:
  - `libssh2_channel_window_read_ex()` `read_avail` argument type.
  - `libssh2_base64_decode()` `datalen` argument type.

- fix possible overflow in `sftp_read()`.

  Ref: 4552c73cd5

- formatting in `wincng.h`.

See warning details in the PR's individual commits.

Cherry-picked from #846
Closes #876

src/agent.c

@@ -131,10 +131,10 @@ agent_connect_unix(LIBSSH2_AGENT *agent)
 }
 
 #define RECV_SEND_ALL(func, socket, buffer, length, flags, abstract) \
-    int rc;                                                          \
     size_t finished = 0;                                             \
                                                                      \
     while(finished < length) {                                       \
+        ssize_t rc;                                                  \
         rc = func(socket,                                            \
                   (char *)buffer + finished, length - finished,      \
                   flags, abstract);                                  \
@@ -170,9 +170,10 @@ agent_transact_unix(LIBSSH2_AGENT *agent, agent_transaction_ctx_t transctx)
 
     /* Send the length of the request */
     if(transctx->state == agent_NB_state_request_created) {
-        _libssh2_htonu32(buf, transctx->request_len);
-        rc = _send_all(agent->session->send, agent->fd,
-                       buf, sizeof buf, 0, &agent->session->abstract);
+        _libssh2_htonu32(buf, (uint32_t)transctx->request_len);
+        rc = (int)_send_all(agent->session->send, agent->fd,
+                            buf, sizeof buf, 0,
+                            &agent->session->abstract);
         if(rc == -EAGAIN)
             return LIBSSH2_ERROR_EAGAIN;
         else if(rc < 0)
@@ -183,8 +184,9 @@ agent_transact_unix(LIBSSH2_AGENT *agent, agent_transaction_ctx_t transctx)
 
     /* Send the request body */
     if(transctx->state == agent_NB_state_request_length_sent) {
-        rc = _send_all(agent->session->send, agent->fd, transctx->request,
-                       transctx->request_len, 0, &agent->session->abstract);
+        rc = (int)_send_all(agent->session->send, agent->fd,
+                            transctx->request, transctx->request_len, 0,
+                            &agent->session->abstract);
         if(rc == -EAGAIN)
             return LIBSSH2_ERROR_EAGAIN;
         else if(rc < 0)
@@ -195,8 +197,9 @@ agent_transact_unix(LIBSSH2_AGENT *agent, agent_transaction_ctx_t transctx)
 
     /* Receive the length of a response */
     if(transctx->state == agent_NB_state_request_sent) {
-        rc = _recv_all(agent->session->recv, agent->fd,
-                       buf, sizeof buf, 0, &agent->session->abstract);
+        rc = (int)_recv_all(agent->session->recv, agent->fd,
+                            buf, sizeof buf, 0,
+                            &agent->session->abstract);
         if(rc < 0) {
             if(rc == -EAGAIN)
                 return LIBSSH2_ERROR_EAGAIN;
@@ -214,8 +217,9 @@ agent_transact_unix(LIBSSH2_AGENT *agent, agent_transaction_ctx_t transctx)
 
     /* Receive the response body */
     if(transctx->state == agent_NB_state_response_length_received) {
-        rc = _recv_all(agent->session->recv, agent->fd, transctx->response,
-                       transctx->response_len, 0, &agent->session->abstract);
+        rc = (int)_recv_all(agent->session->recv, agent->fd,
+                            transctx->response, transctx->response_len, 0,
+                            &agent->session->abstract);
         if(rc < 0) {
             if(rc == -EAGAIN)
                 return LIBSSH2_ERROR_EAGAIN;
@@ -311,7 +315,7 @@ agent_transact_pageant(LIBSSH2_AGENT *agent, agent_transaction_ctx_t transctx)
                        transctx->request_len);
 
     cds.dwData = PAGEANT_COPYDATA_ID;
-    cds.cbData = 1 + strlen(mapname);
+    cds.cbData = (DWORD)(1 + strlen(mapname));
     cds.lpData = mapname;
 
     id = SendMessage(hwnd, WM_COPYDATA, (WPARAM) NULL, (LPARAM) &cds);