Add TCP keepalive support to libpq.

This adds four additional connection parameters to libpq: keepalives,
keepalives_idle, keepalives_count, and keepalives_interval.
keepalives default to on, per discussion, but can be turned off by
specifying keepalives=0.  The remaining parameters, where supported,
can be used to adjust how often keepalives are sent and how many
can be lost before the connection is broken.

The immediate motivation for this patch is to make sure that
walreceiver will eventually notice if the master reboots without
closing the connection cleanly, but it should be helpful in other
cases as well.

Tollef Fog Heen, Fujii Masao, and me.