pgsql: Fix PQencryptPasswordConn to work with older server versions.

Fix PQencryptPasswordConn to work with older server versions.

password_encryption was a boolean before version 10, so cope with "on" and
"off".

Also, change the behavior with "plain", to treat it the same as "md5".
We're discussing removing the password_encryption='plain' option from the
server altogether, which will make this the only reasonable choice, but
even if we kept it, it seems best to never send the password in cleartext.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/20bf7b2b0afcb53608ec37005ee7f831132925d2

Modified Files
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doc/src/sgml/libpq.sgml | 4 +++-
src/interfaces/libpq/fe-auth.c | 21 ++++++++++++++-------
2 files changed, 17 insertions(+), 8 deletions(-)