net: clean up builtin DNS stub resolver, fix tests

net: clean up builtin DNS stub resolver, fix tests

This change does clean up as preparation for fixing #11081.

- renames cfg to resolvConf for clarification
- adds a new type resolverConfig and its methods: init, update,
  tryAcquireSema, releaseSema for mutual exclusion of resolv.conf data
- deflakes, simplifies tests for resolv.conf data; previously the tests
  sometimes left some garbage in the data

Change-Id: I277ced853fddc3791dde40ab54dbd5c78114b78c
Reviewed-on: https://go-review.googlesource.com/10931
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>