iflib: Better control over queue core assignment

By default, cores are now assigned to queues in a sequential
manner rather than all NICs starting at the first core. On a four-core
system with two NICs each using two queue pairs, the nic:queue -> core
mapping has changed from this:

0:0 -> 0, 0:1 -> 1
1:0 -> 0, 1:1 -> 1

To this:

0:0 -> 0, 0:1 -> 1
1:0 -> 2, 1:1 -> 3

Additionally, a device can now be configured to use separate cores for TX
and RX queues.

Two new tunables have been added, dev.X.Y.iflib.separate_txrx and
dev.X.Y.iflib.core_offset. If core_offset is set, the NIC is not part
of the auto-assigned sequence.

Reviewed by: marius
MFC after: 2 weeks
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D20029