Make lload require own sector and lunload own land unit

Make lload require own sector and lunload own land unit

This is for consistency with load and unload.

Before, you could use lunload for allied land units and lload in
allied sectors, but the command failed when stuff was loaded in allied
sectors.  Doing that with lload no longer fails the command, because
the check that does that in load_comm_land() is now masked by the new
checks in lload().  Note that loading with lunload still fails the
command; that inconsistency will be removed in a later commit.

load_plane_land() already refused to load planes from foreign sectors.
This commit makes that check redundant, so remove it.  Functional
change: lload now refuses to load foreign planes silently, unless both
land unit and plane were named by number.  This is consistent with
load.