Move rand.go to rng.go to prepare for some surgery.

R=rsc
APPROVED=rsc
DELTA=637  (318 added, 318 deleted, 1 changed)
OCL=35534
CL=35536

src/pkg/rand/Makefile

@@ -6,6 +6,6 @@ include $(GOROOT)/src/Make.$(GOARCH)
 
 TARG=rand
 GOFILES=\
-	rand.go\
+	rng.go\
 
 include $(GOROOT)/src/Make.pkg