Don't let planes fly to a carrier without sufficient space

Don't let planes fly to a carrier without sufficient space

We test whether the the carrier has space for each plane individually
instead of whether it has space for all of them.  The planes that fit
land, the others abort and get teleported home.  Abusable.

pln_oneway_to_carrier_ok() was created in commit 1127762c (v4.2.17) to
fix almost the same bug.  It worked fine then, because
fit_plane_on_ship() worked with load counters, and incremented them.

Broken in commit 3e370da5 (v4.3.17), which made fit_plane_on_ship()
count the loaded planes, to permit the removal of load counters.  But
unlike load counters, loaded planes don't change during
pln_oneway_to_carrier_ok().  Thus, each plane is checked individually.

Fix by tallying all the planes before checking for space.