pgsql: 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: /messages/by-id/b6fb018b-f05c-4afd-abd3-318c649faf18@highgo.ca

Branch
------
REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6ddc8556ce4aff68446ebe00f14930bc0bbeccca

Modified Files
--------------
src/backend/libpq/be-secure-openssl.c | 9 +++++----
src/interfaces/libpq/fe-secure-openssl.c | 11 ++++++-----
2 files changed, 11 insertions(+), 9 deletions(-)