pgsql: Add settings to control SSL/TLS protocol version

Add settings to control SSL/TLS protocol version

For example:

    ssl_min_protocol_version = 'TLSv1.1'
    ssl_max_protocol_version = 'TLSv1.2'

Reviewed-by: Steve Singer <steve@ssinger.info>
Discussion: https://www.postgresql.org/message-id/flat/1822da87-b862-041a-9fc2-d0310c3da173@2ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e73e67c719593c1c16139cc6c516d8379f22f182

Modified Files
--------------
doc/src/sgml/config.sgml                      |  44 +++++++++
src/backend/libpq/be-secure-openssl.c         | 123 +++++++++++++++++++++++++-
src/backend/libpq/be-secure.c                 |   3 +
src/backend/utils/misc/guc.c                  |  33 +++++++
src/backend/utils/misc/postgresql.conf.sample |   2 +
src/include/libpq/libpq.h                     |  11 +++
6 files changed, 214 insertions(+), 2 deletions(-)