use Mpl and pl; drop Mreg and sr.

Reference: /n/atom/patch/applied/9mcsclean
Date: Sat Jun  7 18:03:03 CES 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/9/port/mcslock.c	Sat Jun  7 18:02:52 2014
+++ /sys/src/9/port/mcslock.c	Sat Jun  7 18:02:53 2014
@@ -147,14 +147,14 @@
 ilock(Lock *l)
 {
 	uintptr pc;
-	Mreg s;
+	Mpl pl;
 	LockEntry *ql;
 
 	pc = getcallerpc(&l);
-	s = splhi();
+	pl = splhi();
 	ql = allocle(l, pc);
 	ql->isilock = 1;
-	ql->sr = s;
+	ql->pl = pl;
 	/* the old taslock code would splx(s) to allow interrupts while waiting (if not nested) */
 	mcslock(l, ql);
 
@@ -168,7 +168,7 @@
 void
 iunlock(Lock *l)
 {
-	Mreg s;
+	Mpl pl;
 	LockEntry *ql;
 
 	if(islo())
@@ -181,12 +181,12 @@
 			m->machno, ql->m->machno, getcallerpc(&l));
 	}
 	mcsunlock(l, ql);
-	s = ql->sr;
+	pl = ql->pl;
 	ql->used = nil;
 	m->ilockdepth--;
 	if(up != nil)
 		up->lastilock = nil;
-	splx(s);
+	splx(pl);
 }
 
 int
--- /sys/src/9/port/portdat.h	Sat Jun  7 18:02:55 2014
+++ /sys/src/9/port/portdat.h	Sat Jun  7 18:02:56 2014
@@ -81,10 +81,7 @@
 	uint	locked;
 	Lock*	used;
 	int	isilock;
-	union {					/* GAK */
-		Mpl	pl;
-		Mreg	sr;
-	};
+	Mpl	pl;
 	/* for debugging */
 	uintptr	pc;
 	Proc*	p;