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.

repmgr.conf.sample

@@ -258,6 +258,11 @@ ssh_options='-q -o ConnectTimeout=10'	# Options to append to "ssh"
 # These settings are only applied when repmgrd is running. Values shown
 # are defaults.
 
+#repmgrd_pid_file=			# Path of PID file to use for repmgrd; if not set, a PID file will
+                                        # be generated in a temporary directory specified by the environment
+                                        # variable $TMPDIR, or if not set, in "/tmp". This value can be overridden
+                                        # by the command line option "-p/--pid-file"; the command line option
+                                        # "--no-pid-file" will force PID file creation to be skipped.
 #failover=manual			# one of 'automatic', 'manual'.
 					# determines what action to take in the event of upstream failure
 					#