connection: fix buffer-overflow in close_fds()

If we push two messages via wl_connection_write() and both messages
contain more than MAX_FDS_OUT file-descriptors combined, then
wl_connection_flush() will write only MAX_FDS_OUT of them, but close all
pending ones, too.

Furthermore, close_fds() will copy more FDs out of the buffer than it can
hold and cause a buffer overflow. Therefore, we simply pass a maximum
limit to close_fds().

During shutdown, we simply close all available FDs.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>