nvmecontrol: New commands to support Fabrics hosts

- discover: Connects to a remote Discovery controller, fetches its
Discovery Log Page, and enumerates the remote controllers described
in the log page.

The -v option can be used to display the Identify Controller data
structure for the Discovery controller. This is only really useful
for debugging.

- connect: Connects to a remote I/O controller and establishes an
association of an admin queue and a single I/O queue. The
association is handed off to the in-kernel host to create a new
nvmeX device.

- connect-all: Connects to a Discovery controller and attempts to
create an association with each I/O controller enumerated in the
Discovery controller's Discovery Log Page.

- reconnect: Establishes a new association with a remote I/O
controller for an existing nvmeX device. This can be used to
restore access to a remote I/O controller after the loss of a prior
association due to a transport error, controller reboot, etc.

- disconnect: Deletes one or more nvmeX devices after detaching its
namespaces and terminating any active associations. The devices to
delete can be identified by either a nvmeX device name or the NQN of
the remote controller.

- disconnect-all: Deletes all active associations with remote
controllers.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44715