onvert vmx(4) to being an iflib driver.

Also, expose IFLIB_MAX_RX_SEGS to iflib drivers and add
iflib_dma_alloc_align() to the iflib API.

Performance is generally better with the tunable/sysctl
dev.vmx.<index>.iflib.tx_abdicate=1.

Reviewed by: shurd
MFC after: 1 week
Relnotes: yes
Sponsored by: RG Nets
Differential Revision: https://reviews.freebsd.org/D18761