multistart: allow tokens as start symbols

After all, why not?

* src/reader.c (switching_token): Use symbol_id_get.
(check_start_symbols): Require that the start symbol is a token only
if it's the only one.
* examples/c/lexcalc/parse.y: Let NUM be a start symbol.

examples/c/lexcalc/parse.y

@@ -79,7 +79,7 @@
 %type <int> exp expression line
 %printer { fprintf (yyo, "%d", $$); } <int>
 
-%start input expression
+%start input expression NUM
 
 // Precedence (from lowest to highest) and associativity.
 %left "+" "-"