pgsql: Teach unix_latch.c to use poll() where available.

Teach unix_latch.c to use poll() where available.

poll() is preferred over select() on platforms where both are available,
because it tends to be a bit faster and it doesn't have an arbitrary limit
on the range of FD numbers that can be accessed. The FD range limit does
not appear to be a risk factor for any 9.1 usages, so this doesn't need to
be back-patched, but we need to have it in place if we keep on expanding
the uses of WaitLatch.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a180776f7a1c4554f214bd9e67bd63bfaf33e339

Modified Files
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src/backend/port/unix_latch.c | 106 +++++++++++++++++++++++++++++++++++++---
1 files changed, 98 insertions(+), 8 deletions(-)