(diridx): New. Use instead of chkdir() where direction characters

(diridx): New.  Use instead of chkdir() where direction characters
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.