Clamp number of vectors to IOV_MAX / UIO_MAXIOV for GOutputStream writev() calls in the implementations

More vectors will give an error and we can simply clamp here and
consider it like a short write instead.

In case of GSocketOutputStream this is done here instead of inside
GSocket before calling sendmsg() because we we can't generically handle
short writes when sending messages on a socket, e.g. for datagram
sockets this causes only part of the datagram to be sent and an error
would be more useful in this case than sending corrupted data.

Also reduce the fallback limit to 16 in gsocket.c as that's the minimum
value required by POSIX and add a static assertion that the limit is
never bigger than G_MAXINT as that's the type recvmmsg/sendmmsg take.

gio/gioprivate.h

@@ -35,6 +35,23 @@ gboolean g_output_stream_async_close_is_via_threads (GOutputStream *stream);
 void g_socket_connection_set_cached_remote_address (GSocketConnection *connection,
                                                     GSocketAddress    *address);
 
+/* POSIX defines IOV_MAX/UIO_MAXIOV as the maximum number of iovecs that can
+ * be sent in one go. We define our own version of it here as there are two
+ * possible names, and also define a fall-back value if none of the constants
+ * are defined */
+#if defined(IOV_MAX)
+#define G_IOV_MAX IOV_MAX
+#elif defined(UIO_MAXIOV)
+#define G_IOV_MAX UIO_MAXIOV
+#else
+/* 16 is the minimum value required by POSIX */
+#define G_IOV_MAX 16
+#endif
+
+/* The various functions taking iovecs as parameter use a plain int
+ * for the number of vectors. Limit it to G_MAXINT for this reason.
+ */
+G_STATIC_ASSERT (G_IOV_MAX <= G_MAXINT);
 
 G_END_DECLS