Added packet-interface level callback that can be used in conjunction
with upcalls to batch receive side actions.  The callback provides
batch start and finish events during receive processing on a given
interface.

Added support to libev to use this mechanism to reduce ev_async_send()
calls under load, which could occur as frequently as once per upcall.

Note that if the application is structured such that traffic from a
single interface is being handled by sockets spread across more than
one event loop, using ev_loop_attach_uinet_interface() will only
provide the ev_async_send() avoidance improvement to one of those
loops.
Improve batching efficiency when there are overlapping batches from
multiple interfaces.
Merge remote-tracking branch 'origin/rx_batch'

Conflicts:
	lib/libuinet/uinet_if_netmap.c