Support tcp_keepalives_idle option on Solaris.

Support tcp_keepalives_idle option on Solaris.

Turns out that the socket option for this is named TCP_KEEPALIVE_THRESHOLD,
at least according to the tcp(7P) man page for Solaris 11.  (But since that
text refers to "SunOS", it's likely pretty ancient.)  It appears that the
symbol TCP_KEEPALIVE does get defined on that platform, but it doesn't
seem to represent a valid protocol-level socket option.  This leads to
bleats in the postmaster log, and no tcp_keepalives_idle functionality.

Per bug #14720 from Andrey Lizenko, as well as an earlier report from
Dhiraj Chawla that nobody had followed up on.  The issue's been there
since we added the TCP_KEEPALIVE code path in commit 5acd417c8, so
back-patch to all supported branches.

Discussion: https://postgr.es/m/20170627163757.25161.528@wrigleys.postgresql.org