Use ppoll(2), if available, to wait for input in pgbench.

Previously, pgbench always used select(2) for this purpose, but that's
problematic for very high client counts, because select() can't deal
with file descriptor numbers larger than FD_SETSIZE.  It's pretty common
for that to be only 1024 or so, whereas modern OSes can allow many more
open files than that.  Using poll(2) would surmount that problem, but it
creates another one: poll()'s timeout resolution is only 1ms, which is
poor enough to cause problems with --rate specifications approaching or
exceeding 1K TPS.

On platforms that have ppoll(2), which includes Linux and recent
FreeBSD, we can use that to avoid the FD_SETSIZE problem without any
loss of timeout resolution.  Hence, add configure logic to test for
ppoll(), and use it if available.

This patch introduces an abstraction layer into pgbench that could
be extended to support other kernel event-wait APIs such as kevents.
But actually adding such support is a matter for some future patch.

Doug Rady, reviewed by Robert Haas and Fabien Coelho, and whacked around
a good bit more by me

Discussion: https://postgr.es/m/23D017C9-81B7-484D-8490-FD94DEC4DF59@amazon.com

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