Mark ssl_passphrase_command as GUC_SUPERUSER_ONLY.

This commit changes the GUC ssl_passphrase_command so that
it's examinable by only superuser and a member of pg_read_all_settings.
Per discussion, we determined to do this because the parameter may
contain a sensitive informtaion like a passphrase itself.

Author: Insung Moon
Reviewed-by: Keisuke Kuroda
Discussion: https://postgr.es/m/CAEMmqBuHVGayc+QkYKgx3gWSdqwTAQGw+0DYn3WhcX-eNa2ntA@mail.gmail.com

src/backend/utils/misc/guc.c

@@ -4268,7 +4268,8 @@ static struct config_string ConfigureNamesString[] =
 	{
 		{"ssl_passphrase_command", PGC_SIGHUP, CONN_AUTH_SSL,
 			gettext_noop("Command to obtain passphrases for SSL."),
-			NULL
+			NULL,
+			GUC_SUPERUSER_ONLY
 		},
 		&ssl_passphrase_command,
 		"",