Define the "identifier" of a symbol.

Define the "identifier" of a symbol.

Symbols may have several string representations, for instance if they
have an alias.  What I call its "id" is a string that can be used as
an identifier.  May not exist.

Currently the symbols which have the "tag_is_id" flag set are those that
don't have an alias.  Look harder for the id.

	* src/output.c (is_identifier): Move to...
	* src/symtab.c (is_identifier): here.
	* src/symtab.h, src/symtab.c (symbol_id_get): New.
	* src/output.c (symbol_definitions_output): Use it to define "id"
	and "has_id".
	Remove the definition of "tag_is_id".
	* data/lalr1.cc: Use the "id" and "has_id" whereever "tag" and
	"tag_is_id" were used to produce code.
	We still use "tag" for documentation.