Use $accept and $end, as BYacc and BTYacc do, instead of $axiom and $.

Use $accept and $end, as BYacc and BTYacc do, instead of $axiom and $.
* src/symtab.h, src/symtab.c (eoftoken, axiom): Rename as...
(endtoken, accept): these.
* src/reader.c (reader): Set endtoken's default tag to "$end".
Set undeftoken's tag to "$undefined" instead of "$undefined.".
* doc/bison.texinfo (Table of Symbols): Mention $accept and $end.
Adjust.