exp/terminal: rename terminal.go to util.go

(This is part of removing the duplicate code between exp/terminal and
exp/ssh, but hg is having a very hard time keeping up so I'm doing it
in small steps.)

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5373061

src/pkg/exp/terminal/Makefile

@@ -9,7 +9,7 @@ GOFILES=\
 	shell.go\
 
 ifneq ($(GOOS),windows)
-GOFILES+=terminal.go
+GOFILES+=util.go
 endif
 
 include ../../../Make.pkg