Fix flying commands not to let planes do double duty as escorts

Fix flying commands not to let planes do double duty as escorts

Commit 7ca4f412 (v4.3.12) marked planes flying a sortie with
PLN_LAUNCHED, and made pln_arm() reject planes with that flag set.
This was designed to reject escorts that were already flying as
bombers.  It didn't work, because the test for PLN_LAUNCHED used a
stale copy of the plane created by pln_sel().  Fix by getting a fresh
copy.

The bug always existed, but the botched fix in commit 7ca4f412 made it
worse.  Before, ac_encounter() dropped escorts that were also bombers,
so the bug merely wasted plane fuel.  After, such planes were
effectively duplicated, and damage to one of them, usually the bomber,
was wiped out.  Abusable.