add support for bpf on "subsystems", not just network interfaces

add support for bpf on "subsystems", not just network interfaces

bpf assumed that it was being unconditionally attached to network
interfaces, and maintained a pointer to a struct ifnet *. this was
mostly used to get at the name of the interface, which is how
userland asks to be attached to a particular interface. this diff
adds a pointer to the name and uses it instead of the interface
pointer for these lookups. this in turn allows bpf to be attached
to arbitrary subsystems in the kernel which just have to supply a
name rather than an interface pointer. for example, bpf could be
attached to pf_test so you can see what packets are about to be
filtered. mpi@ is using this to look at usb transfers.

bpf still uses the interface pointer for bpfwrite, and for enabling
and disabling promisc. however, these are nopped out for subsystems.

ok mpi@