Rename getDirPath to afs_getDirPath in preparation for export

Rename getDirPath to afs_getDirPath in preparation for export

The symbol name getDirPath is rather generic and we probably shouldn't
squat on it in the application's namespace.  In preparation for exporting
this functionality from the Unix shared libraries, rename it to
afs_getDirPath.

Retain a Windows-only wrapper getDirPath that can continue to be
exported from libafsauthent on Windows, for ABI compatibility.
New consumers should use afs_getDirPath.

Change-Id: Ie3f3f7b0662451353834d2e3b5c3dd1131c1935e
Reviewed-on: https://gerrit.openafs.org/13058
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Tested-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>