Centralize compat support for PCIOCGETCONF.

The pre-7.x compat for both native and 32-bit code was already in
pci_user.c. Use this infrastructure to add implement 32-bit support.
This is more correct as ioctl(2) commands only have meaning in the
context of a file descriptor.

Reviewed by: kib
Approved by: re (gjb)
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential revision: https://reviews.freebsd.org/D17324