ctld: Permit simpler syntax for target LUNs in UCL

Allow the LUN number to be specified as the key for a LUN instead
of requiring it as a "number" field. If a key is used, permit
a simple string value to be used for non-anymous LUNs. This permits
replacing:

lun = [

		{ number = 0, name = zvol_lun },
		{
			number = 1
			backend = ramdisk
			size = 1GB
		}

]

with:

lun = {

		0 = zvol_lun
		1 {
			backend = ramdisk
			size = 1GB
		}

}

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D48939