pgsql: Avoid logging complaints about abandoned connections when using

Avoid logging complaints about abandoned connections when using PAM.

For a long time (since commit aed378e8d) we have had a policy to log
nothing about a connection if the client disconnects when challenged
for a password.  This is because libpq-using clients will typically
do that, and then come back for a new connection attempt once they've
collected a password from their user, so that logging the abandoned
connection attempt will just result in log spam.  However, this did
not work well for PAM authentication: the bottom-level function
pam_passwd_conv_proc() was on board with it, but we logged messages
at higher levels anyway, for lack of any reporting mechanism.
Add a flag and tweak the logic so that the case is silent, as it is
for other password-using auth mechanisms.

Per complaint from Yoann La Cancellera.  It's been like this for awhile,
so back-patch to all supported branches.

Discussion: https://postgr.es/m/CACP=ajbrFFYUrLyJBLV8=q+eNCapa1xDEyvXhMoYrNphs-xqPw@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f9bd3b6d929aa731ef5b999c9fadfc2acd0b7cfe

Modified Files
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src/backend/libpq/auth.c | 26 +++++++++++++++++---------
1 file changed, 17 insertions(+), 9 deletions(-)