Support %define "KEY" {VALUE}.

* src/scan-code.h, src/scan-code.l (translate_action)
(translate_rule_action, translate_symbol_action, translate_code):
Return char *, not const char *.
* src/parse-gram.y (declaration): Rename as...
(prologue_declaration): this.
(string_content): Remove this nonterminal, use STRING.
(braceless, content, content.opt): New nonterminal.
Use them.
(%define): Now accept content.opt, i.e., accept also BRACED_CODE
as value.
* src/scan-gram.l (getargs.h): Don't include it.