pgsql: Allow password file name to be specified as a libpq connection p

Allow password file name to be specified as a libpq connection parameter.

Formerly an alternate password file could only be selected via the
environment variable PGPASSFILE; now it can also be selected via a
new connection parameter "passfile", corresponding to the conventions
for most other connection parameters. There was some concern about
this creating a security weakness, but it was agreed that that argument
was pretty thin, and there are clear use-cases for handling password
files this way.

Julian Markwort, reviewed by Fabien Coelho, some adjustments by me

Discussion: https://postgr.es/m/a4b4f4f1-7b58-a0e8-5268-5f7db8e8ccaa@uni-muenster.de

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ba005f193d88a8404e81db3df223cf689d64d75e

Modified Files
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doc/src/sgml/libpq.sgml | 35 +++++++++----
src/interfaces/libpq/fe-auth.c | 5 +-
src/interfaces/libpq/fe-connect.c | 100 +++++++++++++++++++-------------------
src/interfaces/libpq/libpq-int.h | 3 +-
4 files changed, 81 insertions(+), 62 deletions(-)