os: Add epoll-like pollset implementation for AIX

AIX's poll only allows FD_SETSIZE entries in the fd list, which is
insufficient for expanded MaxClients.

As a bonus, x11perf -noop with ~250 xlogos connected is slightly faster
with pollset:

 before          after         Operation
---------   ----------------   --------------------
5750000.0   5990000.0 (1.04)   X protocol NoOperation

Signed-off-by: Peter Harris <pharris@opentext.com>
Acked-by: Keith Packard <keithp@keithp.com>