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/misc.c

@@ -79,7 +79,7 @@ int _libssh2_snprintf(char *cp, size_t cp_max_len, const char *fmt, ...)
     va_start(args, fmt);
     n = vsnprintf(cp, cp_max_len, fmt, args);
     va_end(args);
-    return (n < cp_max_len) ? n : (cp_max_len - 1);
+    return (n < (int)cp_max_len) ? n : (int)(cp_max_len - 1);
 }
 #endif
 
@@ -230,14 +230,14 @@ _libssh2_ntohu32(const unsigned char *buf)
 libssh2_uint64_t
 _libssh2_ntohu64(const unsigned char *buf)
 {
-    unsigned long msl, lsl;
-
-    msl = ((libssh2_uint64_t)buf[0] << 24) | ((libssh2_uint64_t)buf[1] << 16)
-        | ((libssh2_uint64_t)buf[2] << 8) | (libssh2_uint64_t)buf[3];
-    lsl = ((libssh2_uint64_t)buf[4] << 24) | ((libssh2_uint64_t)buf[5] << 16)
-        | ((libssh2_uint64_t)buf[6] << 8) | (libssh2_uint64_t)buf[7];
-
-    return ((libssh2_uint64_t)msl <<32) | lsl;
+    return ((libssh2_uint64_t)buf[0] << 56)
+         | ((libssh2_uint64_t)buf[1] << 48)
+         | ((libssh2_uint64_t)buf[2] << 40)
+         | ((libssh2_uint64_t)buf[3] << 32)
+         | ((libssh2_uint64_t)buf[4] << 24)
+         | ((libssh2_uint64_t)buf[5] << 16)
+         | ((libssh2_uint64_t)buf[6] <<  8)
+         | ((libssh2_uint64_t)buf[7]);
 }
 
 /* _libssh2_htonu32
@@ -245,7 +245,7 @@ _libssh2_ntohu64(const unsigned char *buf)
 void
 _libssh2_htonu32(unsigned char *buf, uint32_t value)
 {
-    buf[0] = (value >> 24) & 0xFF;
+    buf[0] = (unsigned char)((value >> 24) & 0xFF);
     buf[1] = (value >> 16) & 0xFF;
     buf[2] = (value >> 8) & 0xFF;
     buf[3] = value & 0xFF;
@@ -281,7 +281,7 @@ void _libssh2_store_bignum2_bytes(unsigned char **buf,
     for(p = bytes; len > 0 && *p == 0; --len, ++p) {}
 
     extraByte = (len > 0 && (p[0] & 0x80) != 0);
-    _libssh2_store_u32(buf, len + extraByte);
+    _libssh2_store_u32(buf, (uint32_t)(len + extraByte));
 
     if(extraByte) {
         *buf[0] = 0;
@@ -319,6 +319,7 @@ static const short base64_reverse_table[256] = {
  *
  * Decode a base64 chunk and store it into a newly alloc'd buffer
  */
+/* FIXME: datalen, src_len -> size_t */
 LIBSSH2_API int
 libssh2_base64_decode(LIBSSH2_SESSION *session, char **data,
                       unsigned int *datalen, const char *src,
@@ -344,15 +345,15 @@ libssh2_base64_decode(LIBSSH2_SESSION *session, char **data,
             d[len] = (unsigned char)(v << 2);
             break;
         case 1:
-            d[len++] |= v >> 4;
+            d[len++] |= (unsigned char)(v >> 4);
             d[len] = (unsigned char)(v << 4);
             break;
         case 2:
-            d[len++] |= v >> 2;
+            d[len++] |= (unsigned char)(v >> 2);
             d[len] = (unsigned char)(v << 6);
             break;
         case 3:
-            d[len++] |= v;
+            d[len++] |= (unsigned char)v;
             break;
         }
         i++;
@@ -483,7 +484,7 @@ _libssh2_debug_low(LIBSSH2_SESSION * session, int context, const char *format,
     int len, msglen, buflen = sizeof(buffer);
     va_list vargs;
     struct timeval now;
-    static int firstsec;
+    static long firstsec;
     static const char *const contexts[] = {
         "Unknown",
         "Transport",