cmd/yacc: allow token names beginning with _

[rsc]

Yacc accepts:

%token X

but that makes X an exported name, often inappropriate. Using a purely lower-case name
breaks a common yacc style convention: upper case for tokens, lower case for
productions. An attempt to satisfy both constraints with:

%token _X

produces a syntax error - apparently Go's yacc does not allow term names beginning with
(containing?) underscore.

Should change yacc to allow underscore in term names.

[robpike]

Comment 1:

This issue was closed by revision cbc9ab7.

Status changed to Fixed.