ptuser: use the "prname" typedef rather than "char[PR_MAXNAMELEN]"

ptuser: use the "prname" typedef rather than "char[PR_MAXNAMELEN]"

There's a typedef for this type, so use it in function prototypes
rather than the more complicated declarator.

Change-Id: I75a9b4e72201e6b2072dba14185579f9c4aa61be
Reviewed-on: http://gerrit.openafs.org/7858
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>