update: Fix revert to deity and "no civilians" corner cases

update: Fix revert to deity and "no civilians" corner cases

We maintain a few sector invariants in sct_prewrite().  Since the
update bypasses sct_prewrite(), it needs to maintain them itself.  The
two should be consistent.

The update reverts deserted sectors to deity in three places:
do_plague(), populace() and produce_sect().  None of them is
consistent with sct_prewrite().

populace() can revert unowned sectors to deity.  This creates bogus
entries in the "lost" file.  Harmless; messed up when the lost items
were added in 4.0.7.  Visible in tests/smoke/final.xdump.

populace() fails to revert when there are only uw left.  If PLAGUE is
enabled, do_plague() already reverted.  Else, produce_sect() will.
This is the only case where they add value to populace().  Can be
traced back all the way to BSD Empire 1.1.

All three neglect to clear mobility.  Harmless.

Fix populace()'s condition for reverting to deity, and make it clear
mobility.  Drop the reverting from do_plague() and produce_sect().

populace() also resets state that applies to civilians when there are
none: work percentage, loyalty and old owner.  However, it resets on
different conditions than sct_prewrite().  Messed up in Chainsaw;
before, populace() didn't reset at all.

For sectors without military, populace() fails to reset.  This can
happen when the update wipes out civilians and military, say by plague
or fallout.  The now bogus work percentage, loyalty and old owner
persist until sct_prewrite() runs on the next non-update sector
update.  Except old owner is reset correctly by populace() when the
sector reverts to deity.  It doesn't when the owner has a land unit
there.

Most of the time, this doesn't matter, as moving civilians into a
sector without civilians overwrites the sector's work percentage,
loyalty and old owner.  However, airlifting and unloading civilians
fail when the old owner differs from the owner.  Else they adopt the
sector's loyalty and work percentage (bug#49 and bug#255).

Fix populace() to reset any sector without civilians, like
sct_prewrite().

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>