pflog: note why we can't handle this all at run time. [skip ci]

pflog: note why we can't handle this all at run time. [skip ci]

I.e., we could handle *some* differences between pflog types at run time
by looking at the length of the header, but we can't handle all of them,
because the NetBSD, DragonFly BSD, and Darwin headers are all the same
length, and there's at least one way that NetBSD and Darwin pflog files
would have to be dissected differently.