Use actual program name rather than "repmgr" when executing remote commands

It's possible some distribution packages may assign a different name to the
"repmgr" binary (typically appending a version number), so we shouldn't hard-code
into the command string.

However it's reasonable to assume the "repmgr" binary will have the same name
across a replication cluster so we won't engage in any contortions to account
for possible variations.

See: https://github.com/2ndQuadrant/repmgr/issues/323

repmgr-client.c

@@ -2796,7 +2796,7 @@ make_remote_repmgr_path(PQExpBufferData *output_buf, t_node_info *remote_node_re
 {
 	appendPQExpBuffer(output_buf,
 					  "%s -f %s ",
-					  make_pg_path("repmgr"),
+					  make_pg_path(progname()),
 					  remote_node_record->config_file);
 
 }