other. Ensure headers in include/ can be included in any order
(except for econfig-spec.h, which is special). New header types.h to
help avoid inclusion cycles. Sort include directives. Remove some
superflous includes.
flight path prompt now works.
(reco): Pass P_ESC to pln_arm() for consistency with call of pln_sel()
and similar code elsewhere. Believed not to make a difference.
(fly, reco): Use it to prevent flying to a carrier that doesn't have
space for the planes. Previously, planes that did this were
teleported home, which could be abused.
(fly): Fail if pln_sel() comes up empty, just like the other commands
to fly planes.
