nvmft: The in-kernel NVMe over Fabrics controller

This is the server (target in SCSI terms) for NVMe over Fabrics.
Userland is responsible for accepting a new queue pair and receiving
the initial Connect command before handing the queue pair off via an
ioctl to this CTL frontend.

This frontend exposes CTL LUNs as NVMe namespaces to remote hosts.
Users can ask LUNS to CTL that can be shared via either iSCSI or
NVMeoF.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44726