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