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/mbedtls.c

@@ -126,7 +126,7 @@ _libssh2_mbedtls_cipher_init(_libssh2_cipher_ctx *ctx,
     if(!ret)
         ret = mbedtls_cipher_setkey(ctx,
                   secret,
-                  mbedtls_cipher_info_get_key_bitlen(cipher_info),
+                  (int)mbedtls_cipher_info_get_key_bitlen(cipher_info),
                   op);
 
     if(!ret)
@@ -526,11 +526,11 @@ _libssh2_mbedtls_rsa_sha2_verify(libssh2_rsa_ctx * rsactx,
 
 #if MBEDTLS_VERSION_NUMBER >= 0x03000000
     ret = mbedtls_rsa_pkcs1_verify(rsactx,
-                                   md_type, hash_len,
+                                   md_type, (unsigned int)hash_len,
                                    hash, sig);
 #else
     ret = mbedtls_rsa_pkcs1_verify(rsactx, NULL, NULL, MBEDTLS_RSA_PUBLIC,
-                                   md_type, hash_len,
+                                   md_type, (unsigned int)hash_len,
                                    hash, sig);
 #endif
     free(hash);
@@ -558,7 +558,7 @@ _libssh2_mbedtls_rsa_sha2_sign(LIBSSH2_SESSION *session,
 {
     int ret;
     unsigned char *sig;
-    unsigned int sig_len;
+    size_t sig_len;
     int md_type;
     (void)hash_len;
 
@@ -587,11 +587,11 @@ _libssh2_mbedtls_rsa_sha2_sign(LIBSSH2_SESSION *session,
         ret = mbedtls_rsa_pkcs1_sign(rsa,
                                      mbedtls_ctr_drbg_random,
                                      &_libssh2_mbedtls_ctr_drbg,
-                                     md_type, hash_len,
+                                     md_type, (unsigned int)hash_len,
                                      hash, sig);
 #else
         ret = mbedtls_rsa_pkcs1_sign(rsa, NULL, NULL, MBEDTLS_RSA_PRIVATE,
-                                     md_type, hash_len,
+                                     md_type, (unsigned int)hash_len,
                                      hash, sig);
 #endif
     }
@@ -634,8 +634,8 @@ gen_publickey_from_rsa(LIBSSH2_SESSION *session,
     unsigned char *key;
     unsigned char *p;
 
-    e_bytes = mbedtls_mpi_size(&rsa->MBEDTLS_PRIVATE(E));
-    n_bytes = mbedtls_mpi_size(&rsa->MBEDTLS_PRIVATE(N));
+    e_bytes = (int)mbedtls_mpi_size(&rsa->MBEDTLS_PRIVATE(E));
+    n_bytes = (int)mbedtls_mpi_size(&rsa->MBEDTLS_PRIVATE(N));
 
     /* Key form is "ssh-rsa" + e + n. */
     len = 4 + 7 + 4 + e_bytes + 4 + n_bytes;