introducing libssh2_socket_t type and fixed iovec for windows

Steven Van Ingelgem introduces libssh2_socket_t as a generic socket
type to use internally to avoid compiler warnings and mistakes. Also,
the private struct iovec declaration for windows is now made to look
like the POSIX struct does.

src/session.c

@@ -1349,7 +1349,7 @@ libssh2_poll(LIBSSH2_POLLFD * fds, unsigned int nfds, long timeout)
     }
 #elif defined(HAVE_SELECT)
     LIBSSH2_SESSION *session = NULL;
-    int maxfd = 0;
+    libssh2_socket_t maxfd = 0;
     fd_set rfds, wfds;
     struct timeval tv;