pgsql: Back-patch libpq support for TLS versions beyond v1.

Back-patch libpq support for TLS versions beyond v1.

Since 7.3.2, libpq has been coded in such a way that the only SSL protocol
it would allow was TLS v1.  That approach is looking increasingly obsolete.
In commit 820f08cabdcbb899 we fixed it to allow TLS >= v1, but did not
back-patch the change at the time, partly out of caution and partly because
the question was confused by a contemporary server-side change to reject
the now-obsolete SSL protocol v3.  9.4 has now been out long enough that
it seems safe to assume the change is OK; hence, back-patch into 9.0-9.3.

(I also chose to back-patch some relevant comments added by commit
326e1d73c476a0b5, but did *not* change the server behavior; hence, pre-9.4
servers will continue to allow SSL v3, even though no remotely modern
client will request it.)

Per gripe from Jan Bilek.

Branch
------
REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2c2c5f0e02b58d225385f5008fb797a90935cb06

Modified Files
--------------
src/backend/libpq/be-secure.c    |    7 +++++++
src/interfaces/libpq/fe-secure.c |   11 ++++++++++-
2 files changed, 17 insertions(+), 1 deletion(-)