auth: make PGetTokens2 work with 3-char cellnames

auth: make PGetTokens2 work with 3-char cellnames

PGetTokens2 accepts two different types of input:
- an integer 'iterator' to request the nth token set for a user
- a string cellname to request the user's token set for that cell

Unfortunately, it distinguishes between these by assuming if the input
length is sizeof(afs_int32) (4 bytes), it must be an integer.  This
assumption is incorrect if the cellname is three (3) characters long
plus a nul terminator.

The result is that the cellname string is interpreted as a very large
"n"; the subsequent search for the user's "very-large-nth-token" fails,
making it appear that the user has no valid token for this cell.

Improve on this heuristic by double-checking any putative integer input.
If it is actually a 3-character string, then process the input as a
cellname instead.

Introduced by commit 5ec5ad5dcca84e99e5f55987cc4f787cd482fdde 'New
GetToken pioctl'.

While here, add doxygen comments.

Reviewed-on: https://gerrit.openafs.org/13599
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
(cherry picked from commit b0278994826f6bd1dfebc39f26282b8fbdadf1a0)

Change-Id: Ib64749d65f03fc564b1d987b426832442be8d5bd
Reviewed-on: https://gerrit.openafs.org/13679
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>