Factor out a single plane's end of sortie into new pln_put1()
Use it in pln_put() and ac_planedamage().
This changes ac_planedamage() to deal with a destroyed airbase.
Before, aborted planes happily landed there. This bug could not
actually bite, because the code neither yields nor does damage to
potential airbases between checking the landing airbase before takeoff
and aborting planes in ac_planedamage().
It changes pln_put() to cope with dead planes. Before, it made them
land as if they lived, fortunately without ill effects (complaints
about not being able to land were suppressed for dead planes).
ac_planedamage() removes dead planes, but pinflak_planedamage()
doesn't, and these end up in pln_put(). pinflak_planedamage() no
longer has to take shot down planes off their carriers, because
pln_put() now takes care of that.