* src/symtab.h (SUNDEF): New.

* src/symtab.c (bucket_new): Init user_token_number to SUNDEF to
stand for `uninitialized', instead of 0.
* src/reader.c (packsymbols, parse_thong_decl): Adjust.
* src/lex.c (lex): Adjust.
* tests/calc.at (_AT_DATA_CALC_Y): Declare a token for EOF.
Number it 0.
Let yylex return it instead of a plain 0.
Reported by Dick Streefland.