* src/gram.h, src/gram.c (error_token_number): Remove, use

* src/gram.h, src/gram.c (error_token_number): Remove, use
errtoken->number.
* src/reader.c (reader): Don't specify the user token number (2)
for $undefined, as it uselessly prevents using it.
* src/gram.h (token_number_t): Move to...
* src/symtab.h: here.
(state_t.number): Is a token_number_t.
* src/print.c, src/reader.c: Use undeftoken->number instead of
hard coded 2.
(Even though this 2 is not the same as above: the number of the
undeftoken remains being 2, it is its user token number which
might not be 2).
* src/output.c (prepare_tokens): Rename the `maxtok' muscle with
`user_token_number_max'.
Output `undef_token_number'.
* data/bison.simple, data/bison.c++: Use them.
Be sure to map invalid yylex return values to
`undef_token_number'.  This saves us from gratuitous SEGV.
* tests/conflicts.at (Solved SR Conflicts)
(Unresolved SR Conflicts): Adjust.
* tests/regression.at (Web2c Actions): Adjust.