clients: close unused keymap fd

In the simple examples in which keymap is not handled, the open
descriptor has to be properly closed.

After each suspend/resume sequence the keymap is send again to
every client. On client weston-simple-egl the leak causes a
segfault when no more file descriptors can be opened.

Close the file descriptor and lazily copy/paste the comment
already available in simple-dmabuf-v4l.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
(cherry picked from commit 4071225c)