nfs: remote utility class to extract current user

Motivation:
the UnixUtils used by nfs client to extract curren user with the help of
reflection. As com.sun.security.auth.module.UnixSystem is a part of
official OpenJDK, there are no reasons to hide it usage.

Modification:
update nfs client to directly call
com.sun.security.auth.module.UnixSystem.

Result:
less magic in the code.

Acked-by: Paul Millar
Target: master
1 job for master in 1 minute and 16 seconds (queued for 27 seconds)
Status Name Job ID Coverage
  Build
passed build #1067

00:01:16