lib/util Split data_blob_hex_string() into upper and lower

lib/util Split data_blob_hex_string() into upper and lower

Rather than have a repeat of the bugs we found at the plugfest where
hexidecimal strings must be in upper or lower case in particular
places, ensure that each caller chooses which case they want.

This reverts most of the callers back to upper case, as things were
before tridge's patch.  The critical call in the extended DN code is
of course handled in lower case.

Andrew Bartlett