pgsql: Set libpq sslcompression to off by default

Set libpq sslcompression to off by default

Since SSL compression is no longer recommended, turn the default in
libpq from on to off.

OpenSSL 1.1.0 and many distribution packages already turn compression
off by default, so such a server won't accept compression anyway.  So
this will mainly affect users of older OpenSSL installations.

Also update the documentation to make clear that this setting is no
longer recommended.

Discussion: https://www.postgresql.org/message-id/flat/595cf3b1-4ffe-7f05-6f72-f72b7afa7993%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e3bdb2d92600ed45bd46aaf48309a436a9628218

Modified Files
--------------
doc/src/sgml/libpq.sgml                  | 31 ++++++++++++++++++++-----------
src/interfaces/libpq/fe-connect.c        |  2 +-
src/interfaces/libpq/fe-secure-openssl.c |  8 ++++----
3 files changed, 25 insertions(+), 16 deletions(-)