Add an explicit VERBOSE log level

Now it's possible to set the level to VERBOSE and DEBUG independently in
debug builds.

Setting the level to VERBOSE in a release build is equivalent to
setting it to DEBUG.

Test: - build, flash
      - adb shell setprop persist.sys.nw_dns_resolver_log VERBOSE
      - adb logcat | grep libnetd_resolv
Change-Id: Ieafa90fa31e74490203d9d0d8c4f548a68ed3504