syscalls: improve nstat, nfstat, nlstat

Optionally return errors when truncating dev_t, ino_t, and nlink_t.
In the interest of code reuse, use freebsd11_cvtstat() to perform the
truncation and error handling and then convert the resulting struct
freebsd11_stat to struct nstat.

Add missing freebsd32 compat syscalls. These syscalls require
translation because struct nstat contains four instances of struct
timespec which in turn contains a time_t and a long.

Reviewed by: kib