dir: Prototype and function name cleanup

dir: Prototype and function name cleanup

Tidy up typing in the dir package, so that we have a specific type
for a directory file handle.

Also rename all of the functions to afs_dir_* globablly, rather than
just renaming for kernel code.

Change-Id: I6750a8eb9f0606d5debf9d677a92b9c8a63dbcf3
Reviewed-on: http://gerrit.openafs.org/4745
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>