Disable ulimit checking on non-Linux unices.

The code turned out not to port to FreeBSD.  Disable the check,
and assume that FreeBSD admins read the docs.

Thanks to Amatis-51.

limit/limit.go

-// +build windows plan9
+// +build !linux
 
 package limit
 

limit/limit_unix.go

-// +build !windows
-// +build !plan9
+// +build linux
 
 package limit