MEDIUM: mworker: find the server ptr using a CLI prefix

Add a struct server pointer in the mworker_proc struct so we can easily
use it as a target for the mworker proxy.

pcli_prefix_to_pid() is used to find the right PID of the worker
when using a prefix in the CLI. (@master, @#<relative pid> , @<pid>)

pcli_pid_to_server() is used to find the right target server for the
CLI proxy.
diff --git a/include/types/global.h b/include/types/global.h
index a5d948e..4b9d019 100644
--- a/include/types/global.h
+++ b/include/types/global.h
@@ -212,6 +212,7 @@
 	int ipc_fd[2]; /* 0 is master side, 1 is worker side */
 	int relative_pid;
 	int reloads;
+	struct server *srv; /* the server entry in the master proxy */
 	struct list list;
 };