MFC r343291:
Convert 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
Relnotes: yes
Sponsored by: RG Nets
Differential Revision: https://reviews.freebsd.org/D18761

MFC r343301:
Add missing dependency to vmxnet3 Makefile and clean it up a bit otherwise.

MFC r343688:
Fix interrupt index configuration when using MSI interrupts.

When in MSI mode, the device was only being configured with one
interrupt index, but it needs two - one for the actual interrupt and
one to park the tx queue at.

Also clarified comments relating to interrupt index assignment.

Reported by: Yuri Pankov <yuripv@yuripv.net>