ctld: Add support for NVMe over Fabrics

While the overall structure is similar for NVMeoF controllers and
iSCSI targets, there are sufficient differences that NVMe support uses
an alternate configuration syntax.

• In authentication groups, permitted NVMeoF hosts can be allowed by names (NQNs) via "host-nqn" values (similar to "initiator-name" for iSCSI). Similarly, "host-address" accepts permitted host addresses similar to "initiator-portal" for iSCSI.

• A new "transport-group" context enumerates transports that can be used by a group of NVMeoF controllers similar to the "portal-group" context for iSCSI. In this section, the "listen" keyword accepts a transport as well as an address to permit other types of transports besides TCP in the future. The "foreign", "offload", and "redirect" keywords are also not meaningful and thus not supported.

• A new "controller" context describes an NVMeoF I/O controller similar to the "target" context for iSCSI. One key difference here is that "lun" objects are replaced by "namespace" objects. However, a "namespace" can reference a named global lun permitting LUNs to be shared between iSCSI targets and NVMeoF controllers.

NB: Authentication via CHAP is not implemented for NVMeoF.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D48773