Properly unregister OpenSSL callbacks when libpq is done with

Properly unregister OpenSSL callbacks when libpq is done with
it's connection. This is required for applications that unload
the libpq library (such as PHP) in which case we'd otherwise
have pointers to these functions when they no longer exist.

This needs a bit more testing before we can consider a backpatch,
so not doing that yet.

In passing, remove unused functions in backend/libpq.

Bruce Momjian and Magnus Hagander, per report and analysis
by Russell Smith.