Get rid of ship and land unit load counters

Get rid of ship and land unit load counters

Load counters are redundant; they can be computed from the carrier
uids.  Keeping them up-to-date as the carriers change is a pain, and
we never got that quite complete.

Computing load counters straight from the carrier uids every time we
need them would be rather inefficient, but computing them from cargo
lists is not.  So do that.

Remove the load counters: struct shpstr members shp_nplane,
shp_nchoppers, shp_nxlight, shp_nland, and struct lndstr members
lnd_nxlight and lnd_nland.

Don't compute/update load counters in build_ship(), build_land(),
land(), ldump(), load_plane_ship(), load_land_ship(),
load_plane_land(), load_land_land(), lstat(), sdump(), shi(), sstat(),
tend_land(), check_trade(), put_combat(), pln_oneway_to_carrier_ok),
pln_newlanding(), fit_plane_on_ship(), fit_plane_on_land(),
unit_list().

Nothing left in fit_plane_off_ship(), fit_plane_off_land(), so remove
them.

load_land_ship(), load_land_land(), check_trade(), pln_newlanding(),
put_plane_on_ship(), take_plane_off_ship(), put_plane_on_land(),
take_plane_off_land() no longer change the carrier, so don't put it.

Remove functions to recompute the load counters from carrier uids:
count_units(), lnd_count_units(), count_planes(), count_land_planes(),
pln_fixup() and lnd_fixup(), along with the latter two's private
copies of fit_plane_on_ship() and fit_plane_on_land().

New cargo list functions to compute load counts: unit_cargo_count()
and unit_nplane(), with convenience wrappers shp_nplane(),
shp_nland(), lnd_nxlight(), lnd_nland().

Use them to make ship selectors nplane, nchoppers, nxlight, nland
virtual.  They now reflect what is loaded, not how the load uses the
available slots.  This makes a difference when x-light planes or
choppers use plane slots.

Use them to make land unit selectors nxlight and nland virtual.

Use them to get load counts in land(), ldump(), load_plane_ship(),
load_land_ship(), load_plane_land(), load_land_land(), sdump(), shi(),
tend_land(), fit_plane_on_land(), trade_desc(), unit_list().

Rewrite fit_plane_on_ship() and could_be_on_ship() to use
shp_nplane().  could_be_on_ship() now takes load count arguments, as
computed by shp_nplane(), so it can be used for checking against an
existing load as well.