pgsql: Incorporate strerror_r() into src/port/snprintf.c, too.

Incorporate strerror_r() into src/port/snprintf.c, too.

This provides the features that used to exist in useful_strerror()
for users of strerror_r(), too. Also, standardize on the GNU convention
that strerror_r returns a char pointer that may not be NULL.

I notice that libpq's win32.c contains a variant version of strerror_r
that probably ought to be folded into strerror.c. But lacking a
Windows environment, I should leave that to somebody else.

Discussion: https://postgr.es/m/2975.1526862605@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/758ce9b7794845f95473c569155d29fcf0e2751b

Modified Files
--------------
src/include/port.h | 7 ++-
src/interfaces/libpq/fe-auth.c | 6 +--
src/interfaces/libpq/fe-connect.c | 18 +++----
src/interfaces/libpq/fe-lobj.c | 14 +++---
src/interfaces/libpq/fe-misc.c | 2 +-
src/interfaces/libpq/fe-secure-openssl.c | 12 ++---
src/interfaces/libpq/fe-secure.c | 4 +-
src/interfaces/libpq/libpq-int.h | 2 +-
src/port/strerror.c | 85 +++++++++++++++++++++++---------
src/port/thread.c | 27 ----------
src/test/thread/thread_test.c | 5 +-
11 files changed, 99 insertions(+), 83 deletions(-)