fly, para, reco, sail, sweep. Old code also screwed up when
getstring() failed, unless player->aborted. Fixed code is still ugly
and cries for a rewrite.
must be valid. Oopses on bad direction characters.
(pathtoxy, ac_encounter): Bad direction characters used to lead to bad
array subscript and potential disaster.
(pathrange, path): Stop on DIR_STOP as well as on bad direction
characters. This is just for consistency with other code; DIR_STOP
should occur only last in a path here.
(sail_nav_fleet, nav_ship): No change except for the oops.
