pgsql: Introduce a psql "\connect -reuse-previous=on|off" option.

Introduce a psql "\connect -reuse-previous=on|off" option.

The decision to reuse values of parameters from a previous connection
has been based on whether the new target is a conninfo string. Add this
means of overriding that default. This feature arose as one component
of a fix for security vulnerabilities in pg_dump, pg_dumpall, and
pg_upgrade, so back-patch to 9.1 (all supported versions). In 9.3 and
later, comment paragraphs that required update had already-incorrect
claims about behavior when no connection is open; fix those problems.

Security: CVE-2016-5424

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6655c07574f2c966ad17e8cc21e9d399f07266f7

Modified Files
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doc/src/sgml/ref/psql-ref.sgml | 21 ++++----
src/bin/psql/command.c | 109 ++++++++++++++++++++++++++++-------------
src/bin/psql/startup.c | 2 +-
3 files changed, 88 insertions(+), 44 deletions(-)