Windows: build mtafsdir.lib and use it

Windows: build mtafsdir.lib and use it

The executables built in src/vol are pthreaded.  Therefore, they
require a pthread safe version of the src/dir objects.  Instead
of building the dir object files in yet another directory, create
a mtafsdir.lib in src/dir and use it in src/vol, src/tviced and
src/tsalvaged.

Change-Id: I50017bd56ec741af1ff3fc1dd39208e610369088
Reviewed-on: http://gerrit.openafs.org/3693
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>