Equip the skeleton chain with location tracking, runtime trace,

Equip the skeleton chain with location tracking, runtime trace,
pure parser and scanner.
* src/parse-skel.y: Request a pure parser, locations, and prefix
renaming.
(%union): Having several members with the same type does not help
type mismatches, simplify.
(YYPRINT, yyprint): New.
(yyerror): ``Rename'' (there is a #define yyerror skel_error) as...
(skel_error): this.
Handle locations.
* src/scan-skel.l: Adjust to these changes.
* src/skeleton.h (LOCATION_RESET, LOCATION_LINES, LOCATION_STEP)
(LOCATION_PRINT, skel_control_t): New.