ensure rndc modzone succeeds twice for a zone in named.conf

ensure rndc modzone succeeds twice for a zone in named.conf

If a zone is in named.conf, not originally added by rndc addzone,
rndc modzone for that zone succeeds once, but subsequent modzone
attempts fail. This is because do_modzone removes the zone config
from global or view options, but it would fail due to 'not found'
once the config is removed.

The fix is to ensure re-adding the updated zone config to the
global or view options. This also works as a more complete fix
for the issue 85453d3 atempted to solve, ensuring rndc showzone
shows the latest config: it now works for multple attemps of
modzone, and with named that is not built with LMDB.

The change in this commit relies on UNCONST in a few places.
That's not clean, but 'add/mod/delzone' generally seems to
need it (for example, delete_zoneconf uses it to modify the list
of zones). In that sense, this change follows the convention
(for a longer term, there may have to be a better API so that we
can modify config obtions that were once parsed).