Clean up %skeleton and %language priority implementation.

* src/getargs.c (skeleton_prio): Use default_prio rather than 2, and
remove static qualifier because others will soon need to see it.
(language_prio): Likewise.
(getargs): Use command_line_prio rather than 0.
* src/getargs.h (command_line_prio, grammar_prio, default_prio): New
enum fields.
(skeleton_prio): Extern it.
(language_prio): Extern it.
* src/parse-gram.y: Use grammar_prio rather than 1.