pci_user: Rename _old to _freebsd6

The pre-freebsd7 stuff is labeled _old right now. Relabel it as
_freebsd6 instead. No functional change intended. I selected freebsd6
because that's the last version to have the ioctls, although I know
there were changes in the interface for 4, 5 and 6. This broadly batches
what we do with system calls that accumulate changes until we break and
use a new one, we use the last major the system call was in.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D45867