Rename connection parameters to control min/max SSL protocol version in libpq

Rename connection parameters to control min/max SSL protocol version in libpq

The libpq parameters ssl{max|min}protocolversion are renamed to use
underscores, to become ssl_{max|min}_protocol_version.  The related
environment variables still use the names introduced in commit ff8ca5f
that added the feature.

Per complaint from Peter Eisentraut (this was also mentioned by me in
the original patch review but the issue got discarded).

Author: Daniel Gustafsson
Reviewed-by: Peter Eisentraut, Michael Paquier
Discussion: https://postgr.es/m/b319e449-318d-e691-4997-1327e166fcc4@2ndquadrant.com