libpq: Use strerror_r instead of strerror

libpq: Use strerror_r instead of strerror

Commit 453c4687377 introduced a use of strerror() into libpq, but that
is not thread-safe.  Fix by using strerror_r() instead.

In passing, update some of the code comments added by 453c4687377, as
we have learned more about the reason for the change in OpenSSL that
started this.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: Discussion: https://postgr.es/m/b6fb018b-f05c-4afd-abd3-318c649faf18@highgo.ca