libc: always set O_RDWR in fcntl on a socket

For better or worse we have no proper way of handling this right now
but contrib libraries, e.g. glib, use it to determine if they can use
the underlying fd.

Fxies #3265.

repos/libports/src/lib/libc/socket_fs_plugin.cc

@@ -977,7 +977,7 @@ int Socket_fs::Plugin::fcntl(Libc::File_descriptor *fd, int cmd, long arg)
 
 	switch (cmd) {
 	case F_GETFL:
-		return context->fd_flags();
+		return context->fd_flags() | O_RDWR;
 	case F_SETFL:
 		context->fd_flags(arg);
 		return 0;