* Factor out queue selection (epair_select_queue()) and mbuf
preparation (epair_prepare_mbuf()) from epair_menq(). It simplifies
epair_menq() implementation and reduces the amount of dependencies
on the neighbouring epair.
* Use dedicated epair_set_state() instead of 2-lines copy-paste
* Factor out unit selection code (epair_handle_unit()) from
epair_clone_create(). It simplifies the clone creation logic.
MFC after: 2 weeks