funopen(): change seekfn argument to use off_t, not fpos_t

funopen(): change seekfn argument to use off_t, not fpos_t
On BSD, fpos_t is typedef'd to off_t but some systems use a struct.
This means fpos_t is not a portable function argument or return value.
Both FreeBSD and the Linux libbsd funopen() have switched to off_t
for this--we should too.  From Joe Nelson.  OK deraadt@