pflog: create bpf tapping points without ifnet(9)

Just as was done for ipfw's log device stop creating entire struct
ifnet's for pflog devices. Create only a bpf_t.

This does mean we lose the create/destroy infrastructure provided by the
clone interface. Rather than implement this ourselves we allow users to
configure the number of pflog interfaces using the net.pflog.if_count
sysctl. We default to 8 devices, but allow up to 256.

The /etc/rc.d/pflog script will create extra devices as required.

While it was possible to rename pflog devices pfctl expected the pflogX
name, so it's safe to assume users never did this.

Requested by: glebius
Reviewed by: glebius
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D57851