MEDIUM: global: remove the relative_pid from global and mworker

The relative_pid is always 1. In mworker mode we also have a
child->relative_pid which is always equalt relative_pid, except for a
master (0) or external process (-1), but these types are usually tested
for, except for one place that was amended to carefully check for the
PROC_O_TYPE_WORKER option.

Changes were pretty limited as most usages of relative_pid were for
designating a process in stats output and peers protocol.
diff --git a/src/peers.c b/src/peers.c
index 958404c..bb68a8e 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -567,7 +567,7 @@
 	min_ver = (peer->flags & PEER_F_DWNGRD) ? PEER_DWNGRD_MINOR_VER : PEER_MINOR_VER;
 	/* Prepare headers */
 	ret = snprintf(msg, size, PEER_SESSION_PROTO_NAME " %u.%u\n%s\n%s %d %d\n",
-	              PEER_MAJOR_VER, min_ver, peer->id, localpeer, (int)getpid(), relative_pid);
+	              PEER_MAJOR_VER, min_ver, peer->id, localpeer, (int)getpid(), 1);
 	if (ret >= size)
 		return 0;