Move symbols handling code out of the reader.

Move symbols handling code out of the reader.
* src/reader.h, src/reader.c (errtoken, undeftoken, eoftoken)
(axiom): Move to...
* src/symtab.h, src/symtab.c: here.
* src/gram.c (start_symbol): Remove: use startsymbol->number.
* src/reader.c (startval): Rename as...
* src/symtab.h, src/symtab.c (startsymbol): this.
* src/reader.c: Adjust.
* src/reader.c (symbol_check_defined, symbol_make_alias)
(symbol_check_alias_consistence, symbol_pack, symbol_translation)
(token_translations_init)
Move to...
* src/symtab.c: here.
* src/reader.c (packsymbols): Move to...
* src/symtab.h, src/symtab.c (symbols_pack): here.
* src/symtab.h, src/symtab.c (symbol_make_alias): Takes SYMVAL as
argument.