Be type correct in osi_ThreadUnique() for FBSD

Be type correct in osi_ThreadUnique() for FBSD

Formerly, in AFS_FBSD50_ENV, we used curproc for the ThreadUnique
value; however, curproc (#defined as curthread->td_proc) is a
struct proc *, not an actual pid.  (As such, it suffers from
a 32/64-bit mismatch on 64-bit systems.)  Use the correct value,
curproc->p_pid, instead.

Change-Id: If07ef3b0814c0fee63b62d2f36fa94d6e45573b9
Reviewed-on: http://gerrit.openafs.org/1528
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>