add portable LIBSSH2_SOCKET_CLOSE() macro

Add `LIBSSH2_SOCKET_CLOSE()` to the public `libssh2.h` header, for user
code. It translates to `closesocket()` on Windows and `close()` on other
platforms.

Use it in example code.

It makes them more readable by reducing the number of `_WIN32` guards.

Closes #1278

example/sftp_nonblock.c

@@ -288,11 +288,7 @@ shutdown:
 
     if(sock != LIBSSH2_INVALID_SOCKET) {
         shutdown(sock, 2);
-#ifdef _WIN32
-        closesocket(sock);
-#else
-        close(sock);
-#endif
+        LIBSSH2_SOCKET_CLOSE(sock);
     }
 
     fprintf(stderr, "all done\n");