Convert `fd_cmask' and `fd_refcnt' types from u_short to 32 bit types.

Convert `fd_cmask' and `fd_refcnt' types from u_short to 32 bit types.

`fd_cmask' and `fd_refcnt' are 16 bit variables which are protected by
different locks and could be not MP independent on all architectures.
`fd_cmask' modifications already protected by fd_lock' rwlock(9), but
actually we do all access to both variables with kernel lock held. So
convert them both before make `fd_cmask' access without kernel when
umask(2) will be unlocked.

Proposed by bluhm@.

ok deraadt@ bluhm@