Backported from X.org:

 commit 0c41b7af4ab0c8d22b88f201293f59524d1e7317
 Author: Keith Packard <keithp@keithp.com>
 Date:   Wed Nov 11 22:02:02 2015 -0800

    os: Add NotifyFd interfaces

    This provides a callback-based interface to monitor file
    descriptors beyond the usual client and device interfaces.

    Modules within the server using file descriptors for reading and/or
    writing can call

        Bool SetNotifyFd(int fd, NotifyFdProcPtr notify_fd, int mask, void *data);

    mask can be any combination of X_NOTIFY_READ and X_NOTIFY_WRITE.

    When 'fd' becomes readable or writable, the notify_fd function will be
    called with the 'fd', the ready conditions and 'data' values as arguments,

    When the module no longer needs to monitor the fd, it will call

        void RemoveNotifyFd(int fd);

    RemoveNotifyFd may be called from the notify function.
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>