With using bind-check-interval, zones are automatically reloaded once
their mtimes change. However, it's also possible to have $INCLUDE
statements in a zonefile, and those are not chased. This means that one
needs to touch the zone file itself to trigger a reload if you just
change one of the included files.
It'd be nice if the bind backend would maintain a list of files that
were read during parsing of a zone file, and monitor them all for
changes. I do realize though that normally the master file would be
updated anyway for the serial, but in certain in-house situations that
is not otherwise required.