Parse and then mostly ignore "signed" qualifier.

Just issue errors if both "signed" and "unsigned" are specified,
or if "signed" is applied to a non-int type.

lex.ll

@@ -117,6 +117,7 @@ reference { Error(*yylloc, "\"reference\" qualifier is no longer supported; "
                            "instead."); }
 return { return TOKEN_RETURN; }
 soa { return TOKEN_SOA; }
+signed { return TOKEN_SIGNED; }
 sizeof { return TOKEN_SIZEOF; }
 static { return TOKEN_STATIC; }
 struct { return TOKEN_STRUCT; }