Improve update-l2database.pl

* add & use adapted version of Net::SNMP::Mixin::Util::normalize_mac()
  (needed because one stray MAC was outputing as ASCII, untranslated)
* delete old copy of update-l2database.pl from sflow directory
* copy comment about testing status on switches from old sflow version,
  and add Extreme BD-8806 to list
* apply $do_nothing to *all* DB-affecting actions
* improve debug output layout and increase output
* replace a printf occurrence with print
* standardise whitespace