libpq: Make SSL errorhandling in backend threadsafe

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, and the call to
strerror need to be replaced with strerror_r which use an allocated
buffer passed back instead of the static buffer.

  * The callback use a new member in the Port struct for passing
    the error detail string, and the Port struct is in turn passed
    as private data in the SSL object
  * The strerror call is replaced with a strerror_r and the static
    errbuf buffer is palloced with the caller being responsible for
    cleanup.

Author: Daniel Gustafsson <[email protected]>
Reviewed-by: Peter Eisentraut <[email protected]>
Discussion: https://postgr.es/m/[email protected]

fixup

Author: danielgustafsson