pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f0256c774daa0dac96154e7ddf54197fb2b83f4d

Modified Files
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src/backend/libpq/pqcomm.c | 40 ++++++++++++++++++++++++++++++---------
src/interfaces/libpq/fe-connect.c | 23 ++++++++++++++++------
2 files changed, 48 insertions(+), 15 deletions(-)