Factor reader_list management out of wait_for_reply.
Factor reader_list management out of wait_for_reply.

Later patches will insert reader_list entries from other entry points.

Signed-off-by: Jamey Sharp <167c1dcbbc9c320e8e02a0c15c886204b4e658a4@minilop.net>
Reviewed-by: Josh Triplett <c028c213ed5efcf30c3f4fc7361dbde0c893c5b7@freedesktop.org>

stapelberg/libxcb,stapelberg/libxcb,stapelberg/libxcb