usnic: move some defines into btl_usnic_util.h

Move MACLEN and IPV4LEN into _util.h and rename them to be MACSTRLEN
and IPV4STRLEN, respectively.

cmr=v1.8.2:ticket=#4734
usnic: remove "device" and "port" language from show_help() messages

Move away from verbs-specific terms "device" and "port" in the usnic
BTL help messages.  Replace them with "usNIC interface" (since usNIC
has no concept of a port).

cmr=v1.8.2:ticket=#4734