Add parameters for controlling locations of server-side SSL files

This allows changing the location of the files that were previously
hard-coded to server.crt, server.key, root.crt, root.crl.

server.crt and server.key continue to be the default settings and are
thus required to be present by default if SSL is enabled.  But the
settings for the server-side CA and CRL are now empty by default, and
if they are set, the files are required to be present.  This replaces
the previous behavior of ignoring the functionality if the files were
not found.

src/backend/libpq/hba.c

@@ -1417,7 +1417,7 @@ parse_hba_auth_opt(char *name, char *val, HbaLine *hbaline, int line_num)
 				ereport(LOG,
 						(errcode(ERRCODE_CONFIG_FILE_ERROR),
 						 errmsg("client certificates can only be checked if a root certificate store is available"),
-						 errhint("Make sure the root.crt file is present and readable."),
+						 errhint("Make sure the configuration parameter \"ssl_ca_file\" is set."),
 				   errcontext("line %d of configuration file \"%s\"",
 							  line_num, HbaFileName)));
 				return false;