Add configuration parameter "repmgr_bindir"

This is to facilitate remote invocation of repmgr when the repmgr
binary is located somewhere other than the PostgreSQL binary directory, as it
cannot be assumed all package maintainers will install repmgr there.

This parameter is optional; if not set (the default), repmgr will fall back
to "pg_bindir" (if set).

Addresses GitHub #246.

repmgr.conf.sample

@@ -147,7 +147,11 @@
 					# *NOTE* "pg_bindir" is only used when repmgr directly
 					# executes PostgreSQL binaries; any user-defined scripts
 					# *must* be specified with the full path
-					#
+
+#repmgr_bindir=''			# Path to repmgr binary directory (location of the repmgr
+					# binary. Only needed if the repmgr executable is not in
+                                        # the system $PATH or the path defined in "pg_bindir".
+
 #use_primary_conninfo_password=false	# explicitly set "password" in recovery.conf's
 					# "primary_conninfo" parameter using the value contained
 					# in the environment variable PGPASSWORD