Separate the parts specific to the SSL library from the general logic.

The previous code structure was

open_client_SSL()
calls verify_peer_name_matches_certificate()
calls verify_peer_name_matches_certificate_name()
calls wildcard_certificate_match()

and was completely in fe-secure-openssl.c.  The new structure is

open_client_SSL() [openssl]
calls pq_verify_peer_name_matches_certificate() [generic]
calls pgtls_verify_peer_name_matches_certificate_guts() [openssl]
calls openssl_verify_peer_name_matches_certificate_name() [openssl]
calls pq_verify_peer_name_matches_certificate_name() [generic]
calls wildcard_certificate_match() [generic]

Move the generic functions into a new file fe-secure-common.c, so the
calls generally go fe-connect.c -> fe-secure.c -> fe-secure-${impl}.c ->
fe-secure-common.c, although there is a bit of back-and-forth between
the last two.
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>