pgsql: Second try at fixing tcp_keepalives_idle option on Solaris.

Second try at fixing tcp_keepalives_idle option on Solaris.

Buildfarm evidence shows that TCP_KEEPALIVE_THRESHOLD doesn't exist
after all on Solaris < 11. This means we need to take positive action to
prevent the TCP_KEEPALIVE code path from being taken on that platform.
I've chosen to limit it with "&& defined(__darwin__)", since it's unclear
that anyone else would follow Apple's precedent of spelling the symbol
that way.

Also, follow a suggestion from Michael Paquier of eliminating code
duplication by defining a couple of intermediate symbols for the
socket option.

In passing, make some effort to reduce the number of translatable messages
by replacing "setsockopt(foo) failed" with "setsockopt(%s) failed", etc,
throughout the affected files. And update relevant documentation so
that it doesn't claim to provide an exhaustive list of the possible
socket option names.

Like the previous commit (f0256c774), back-patch to all supported branches.

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

Branch
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REL9_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e97b7372bf94e20df41b73f81c53157d53826535

Modified Files
--------------
doc/src/sgml/config.sgml | 19 ++++----
doc/src/sgml/libpq.sgml | 19 ++++----
src/backend/libpq/pqcomm.c | 98 ++++++++++++++++-----------------------
src/interfaces/libpq/fe-connect.c | 60 ++++++++++++------------
4 files changed, 89 insertions(+), 107 deletions(-)