Fix some bugs found while fixing the representation and translation
of 64-bit dev_t's (but not ones involving dev_t's).

st_size was supposed to be clamped in cvtstat() and linux's copy_stat(),
but the clamping code wasn't aware that st_size is signed, and also had
an obfuscated off-by-1 value for the unsigned limit, so its effect was
to produce a bizarre negative size instead of clamping.

Change freebsd32's copy_ostat() to be no worse than cvtstat(). It was
missing clamping and bzero()ing of padding.

Reviewed by: kib (except a final fix of the clamp to the signed maximum)