repmgrd: create pid file by default

Traditionally repmgrd will only write a pidfile if explicitly requested with
-p/--pid-file. However it's normally desirable to have a pidfile, and it's
preferable to have one used by default to prevent accidentally starting a second
repmgrd instance.

Following changes made:

 - add configuration file parameter "repmgrd_pid_file" (initially overridden by
   -p/--pid-file for backwards compatibility, though eventually we'll want to
   drop -p/--pid-file altogether)
 - add command line option --no-pid-file
 - if neither "repmgrd_pid_file" nor -p/--pid-file is set, create the pid file
   in a temporary directory

Implements GitHub #457.

configfile.c

@@ -359,6 +359,7 @@ _parse_config(t_configuration_options *options, ItemList *error_list, ItemList *
 	options->async_query_timeout = DEFAULT_ASYNC_QUERY_TIMEOUT;
 	options->primary_notification_timeout = DEFAULT_PRIMARY_NOTIFICATION_TIMEOUT;
 	options->repmgrd_standby_startup_timeout = -1; /* defaults to "standby_reconnect_timeout" if not set */
+	memset(options->repmgrd_pid_file, 0, sizeof(options->repmgrd_pid_file));
 
 	/*-------------
 	 * witness settings
@@ -604,6 +605,8 @@ _parse_config(t_configuration_options *options, ItemList *error_list, ItemList *
 			options->primary_notification_timeout = repmgr_atoi(value, name, error_list, 0);
 		else if (strcmp(name, "repmgrd_standby_startup_timeout") == 0)
 			options->repmgrd_standby_startup_timeout = repmgr_atoi(value, name, error_list, 0);
+		else if (strcmp(name, "repmgrd_pid_file") == 0)
+			strncpy(options->repmgrd_pid_file, value, MAXPGPATH);
 
 		/* witness settings */
 		else if (strcmp(name, "witness_sync_interval") == 0)