Use long for the last argument to VOP_PATHCONF rather than a register_t.

pathconf(2) and fpathconf(2) both return a long. The kern_[f]pathconf()
functions now accept a pointer to a long value rather than modifying
td_retval directly. Instead, the system calls explicitly store the
returned long value in td_retval[0].

Requested by: bde
Reviewed by: kib
Sponsored by: Chelsio Communications