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.

repmgrd.h

@@ -10,6 +10,8 @@
 #include <time.h>
 #include "portability/instr_time.h"
 
+#define OPT_NO_PID_FILE                  1000
+
 extern volatile sig_atomic_t got_SIGHUP;
 extern MonitoringState monitoring_state;
 extern instr_time degraded_monitoring_start;
@@ -26,4 +28,6 @@ const char *print_monitoring_state(MonitoringState monitoring_state);
 
 void		update_registration(PGconn *conn);
 void		terminate(int retval);
+
+
 #endif							/* _REPMGRD_H_ */