ptserver: Allow longer foreign user names

ptserver: Allow longer foreign user names

The ptserver was restricting all names to be less than PR_MAXNAMELEN -
pr_realmNameLen - 1, so that the local realm and a '@' can be safely
appended for foreign user registration. This check is overly
restrictive for foreign users, since the local realm will never be
appended to the user name (it already has the foreign realm name
appended). So, just check for PR_MAXNAMELEN for foreign users.

Original issue reported by Gunnar Krull.

Change-Id: I34c5d908cc2c1b3abe87966115e6ada74c9a93a9
Reviewed-on: http://gerrit.openafs.org/2485
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>