Windows: build UNIX asetkey

With roken and friends it is now possible to build the UNIX
asetkey on Windows.  Do so to reduce duplicate code.

Change-Id: Ic7e050b04e6bed7fe2106de1a163d9ae1fde9efc
Reviewed-on: http://gerrit.openafs.org/10070
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>