os: Implement support for NotifyFd X_NOTIFY_WRITE and removal of AddEnabledDevices/RemoveEnabledDevices

 Backported from X.org:

 commit be5a513fee6cbf29ef7570e57eb0436d70fbd88c
 Author: Keith Packard <keithp@keithp.com>
 Date:   Mon Dec 7 15:12:14 2015 -0800

    Remove AddEnabledDevice and AddGeneralSocket APIs

    All uses of these interfaces should instead be using the NotifyFd API
    instead.
Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>

 commit 4020aacd1fc5b9c63369f011aeb9120af9c55218
 Author: Keith Packard <keithp@keithp.com>
 Date:   Wed Nov 11 22:02:03 2015 -0800

    os: Implement support for NotifyFd X_NOTIFY_WRITE

    This adds the ability to be notified when a file descriptor is
    available for writing.
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
Backported-to-NX-by: Mike Gabriel <mike.gabriel@das-netzwerkteam.de>