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.