PAG numbers are unsigned (and other warnings)

PAG numbers are unsigned (and other warnings)

PAG numbers are represented by afs_uint32. Fix assorted call sites to
reflect this.

While we're here, prototype ktc_newpag, and fix the casts used by
the ktc_curpag() call.

Reviewed-on: http://gerrit.openafs.org/167
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>