Use error message wordings for permissions checks on .pgpass and SSL private

key files that are similar to the one for the postmaster's data directory
permissions check.  (I chose to standardize on that one since it's the most
heavily used and presumably best-wordsmithed by now.)  Also eliminate explicit
tests on file ownership in these places, since the ensuing read attempt must
fail anyway if it's wrong, and there seems no value in issuing the same error
message for distinct problems.  (But I left in the explicit ownership test in
postmaster.c, since it had its own error message anyway.)  Also be more
specific in the documentation's descriptions of these checks.  Per a gripe
from Kevin Hunter.