md_events: disambiguate mdomain from domain
md_events: disambiguate mdomain from domain

When dns-01 callbacks occur, "domain" is the dns domain being
tested.  We also need the mdomain (the certificate's CN), which
is how we find configuration.

Fixes initial config file written by install.

Move sample DNS Update handlers to global config since in
many cases they don't care about the mdomain.  (Of course,
if they need mdomain-specific config, they can go there.)
It's an excuse to demonstrate "inheritance".

icing/mod_md,icing/mod_md,icing/mod_md,icing/mod_md,icing/mod_md