Refactor client-side SSL certificate checking code

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>

src/interfaces/libpq/fe-secure-common.h

@@ -0,0 +1,26 @@
+/*-------------------------------------------------------------------------
+ *
+ * fe-secure-common.h
+ *
+ * common implementation-independent SSL support code
+ *
+ * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/interfaces/libpq/fe-secure-common.h
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#ifndef FE_SECURE_COMMON_H
+#define FE_SECURE_COMMON_H
+
+#include "libpq-fe.h"
+
+extern int pq_verify_peer_name_matches_certificate_name(PGconn *conn,
+											 const char *namedata, size_t namelen,
+											 char **store_name);
+extern bool pq_verify_peer_name_matches_certificate(PGconn *conn);
+
+#endif							/* FE_SECURE_COMMON_H */