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.h

@@ -131,6 +131,7 @@ typedef struct
 	int			async_query_timeout;
 	int			primary_notification_timeout;
 	int			repmgrd_standby_startup_timeout;
+	char		repmgrd_pid_file[MAXPGPATH];
 
 	/* BDR settings */
 	bool		bdr_local_monitoring_only;
@@ -196,7 +197,7 @@ typedef struct
         false, -1, \
 		DEFAULT_ASYNC_QUERY_TIMEOUT, \
 		DEFAULT_PRIMARY_NOTIFICATION_TIMEOUT,	\
-		-1,	\
+		-1, "", \
 		/* BDR settings */ \
 		false, DEFAULT_BDR_RECOVERY_TIMEOUT, \
 		/* service settings */ \