Fix automatic supply of defending and reacting units

Fix automatic supply of defending and reacting units

Being in supply is relevant for defending and reacting units.  The
code used has_supply() to check that.

Contrary to its name, has_supply() does not check whether the land
unit has enough supplies to be in supply, but whether it has or could
draw enough.  So, defending and reacting units did not actually draw
any missing supplies.

Fix that in get_dlist() and att_reacting_units() by calling
resupply_all(), then checking with new lnd_in_supply() instead of
has_supply().  The fix of att_reacting_units() is complicated by the
fact that it is also used in the strength command, and should keep not
drawing supplies there.

Rename has_supply() to lnd_could_be_supplied().  Replace its uses
immediately after resupply_all() by lnd_in_supply().