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.

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>
(cherry picked from commit 554c38473d1465af4c4613209229c274807fffd8)

Change-Id: If527d3f6d619e8bc41914353d28d5d74cb683637
Reviewed-on: https://gerrit.openafs.org/13145
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>