Add /etc/repmgr.conf as a default configuration file location

Also refactor configuration file handling while we're at it.

Previously a configuration file would be ignored if it couldn't
be opened, however that is now treated as an error.

log.c

@@ -248,9 +248,9 @@ logger_init(t_configuration_options * opts, const char *ident)
 	}
 
 	return true;
-
 }
 
+
 bool
 logger_shutdown(void)
 {