split ether_output into resolution, encapsulation, and output functions

if if_output can be overridden on ethernet interfaces, it will allow
things like vlan to do it's packet encapsulation during output
before putting the packet directly on the underlying interface for
output.

this has two benefits. first, it can avoid having ether_output on
pseudo interfaces recurse, which makes profiling of the network
stack a lot clearer. secondly, and more importantly, it allows
pseudo ethernet interface packet encapsulation to by run concurrently
by the stack, rather than having packets unnecessarily serialied
by an ifq.

this diff just splits ether_output up, it doesnt have any interface
take advantage of it yet.

tweaks and ok claudio@

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