PATCH: pgbench - option to build using ppoll() for larger connection counts

[trying again for 2018-01]

This patch enables building pgbench to use ppoll() instead of select()
to allow for more than (FD_SETSIZE - 10) connections. As implemented,
when using ppoll(), the only connection limitation is system resources.

The patch has been implemented to introduce a minimal of #ifdef/#ifndef
clutter in the code.

Without this patch, one is limited to '(FD_SETSIZE - 10)’ number of connections.
Example of something that fails without this patch but works with the patch:

Without the patch:

$ pgbench -j 3000 -c 1500
invalid number of clients: "1500"

With the patch:

$ pgbench -j 3000 -c 1500
starting vacuum...end.
transaction type: <builtin: TPC-B (sort of)>
scaling factor: 2000
query mode: simple
number of clients: 1500
number of threads: 1500
number of transactions per client: 10
number of transactions actually processed: 15000/15000
latency average = 631.730 ms
tps = 2374.430587 (including connections establishing)
tps = 4206.524986 (excluding connections establishing)

doug
--
Doug Rady
Amazon Aurora, RDS PostgreSQL
radydoug(at)amazon(dot)com