Drop the arptype-based nm_utils_hwaddr funcs, and rename the
length-based ones to no longer have _len in their names. This also
switches nm_utils_hwaddr_atoba() to using a length rather than an
arptype, and adds a length argument to nm_utils_hwaddr_valid() (making
nm_utils_hwaddr_valid() now a replacement for nm_utils_hwaddr_aton()
in some places, where we were only using aton() to do validity
checking).