libpq: threadsafety for SSL certificate callback

libpq: threadsafety for SSL certificate callback

In order to make the errorhandling code in backend libpq be thread-
safe the global variable used by the certificate verification call-
back need to be replaced with passing private data.

This moves the threadsafety needle a little but forwards, the call
to strerror_r also needs to be replaced with the error buffer made
thread local.  This is left as future work for when add the thread
primitives required for this to the tree.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/353226C7-97A1-4507-A380-36AA92983AE6@yesql.se