Rename PQsetSSLKeyPassHook and friends

Rename PQsetSSLKeyPassHook and friends

4dc6355210 provided a way for libraries and clients to modify how libpq
handles client certificate passphrases, by installing a hook. However,
these routines are quite specific to how OpenSSL works, so it's
misleading and not future-proof to have these names not refer to OpenSSL.
Change all the names to add "_OpenSSL" after "Hook", and fix the docs
accordingly.

Author: Daniel Gustafsson

Discussion: https://postgr.es/m/981DE552-E399-45C2-9F60-3F0E3770CC61@yesql.se