Centralize ssl tests' check for whether we're using LibreSSL.

Centralize ssl tests' check for whether we're using LibreSSL.

Right now there's only one caller, so that this is merely
an exercise in shoving code from one module to another,
but there will shortly be another one.  It seems better to
avoid having two copies of this highly-subject-to-change test.

Back-patch to v15, where we first introduced some tests that
don't work with LibreSSL.

Reported-by: Thomas Munro <thomas.munro@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: http://postgr.es/m/CA+hUKG+fLqyweHqFSBcErueUVT0vDuSNWui-ySz3+d_APmq7dw@mail.gmail.com
Backpatch-through: 15