Xlib has several independent tasks that need to be performed with the
display unlocked. It does this by replacing the existing sync handler with
one of a variety of internal sync handlers. However, if multiple internal
sync handlers need to run, then the last one registering wins and
previously registered internal sync handlers are never invoked. This
manifested as a bug with DRI applications on Xlib/XCB as that requires
both an XID handler after every XID allocation, and the periodic sequence
number handler. The XID handler would win, and the sequence number handler
would never be invoked.

Fix this by unifying the internal sync handler mechanism into a single
function that calls all of the known internal sync handlers. They all need
to deal with being called when not strictly necessary now.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Jamey Sharp <jamey@minilop.net>
Signed-off-by: Josh Triplett <josh@freedesktop.org>