Bison does not directly support multiple entrypoints

[andreasabel]

The C backend generates code for several entrypoints, but bison allows only one start symbol. Currently, the generated code for the other entrypoints is superfluous and maybe confusing.

Maybe several entrypoints could be implemented using this workaround: https://www.gnu.org/software/bison/manual/html_node/Multiple-start_002dsymbols.html