Remove pr_PrintEntry from public headers

Remove pr_PrintEntry from public headers

pr_PrintEntry isn't used outside of the ptserver directory, and requires
struct prentry, which is defined internally, so remove it from ptuser.h,
and create a new internal header to hold it.

Also, tidy up some type mismatch warnings by making static strings const
char *

Reviewed-on: http://gerrit.openafs.org/22
Reviewed-by: Russ Allbery <rra@stanford.edu>
Verified-by: Russ Allbery <rra@stanford.edu>