Don't hide the ugly fact that default state in '-f' mode is always state 0.

Don't hide the ugly fact that default state in '-f' mode is always state 0.

In '-f' mode, state dispatch generation can be triggered in two ways.
Both ways use 're2c::OutputFile::insert_state_goto', which generates
state dispatch only if it hasn't been already generated. The two ways
are:
1. Explicitly, using '/*!getstate:re2c*/'. In this case default state
   must be state 0 because it's hardcoded in the invocation of
   're2c::OutputFile::insert_state_goto' in 'src/parse/scanner_lex.re'.
2. Implicitly, in 're2c::DFA::emit'. In this case default state must
   be state 0 because if 'prolog_label' is not 0, it means that it's
   not the first time 're2c::DFA::emit' is called and state dispatch
   has already been generated.

This commit makes it explicit that re2c always uses state 0.

Note:
    Currently in '-f' mode re2c generates one global dispatch for
    the whole file (the enumeration of yyFillLabel's is also global).
    All re2c blocks share the same state dispatch, so in '-f' mode all
    re2c blocks must reside in the same function and must be parts of
    the same lexer (exception: in '-r' mode re2c generates one state
    dispatch per use block).

    This is clearly an ugly limitation: one is forced put disconnected
    lexers in different files in '-f' mode.

    Now re2c provides conditions as a way to express related blocks,
    so if users used multiple blocks only for unrelated lexers, we
    could safely limit the scope of state dispatch to a single block.
    But conditions can conflict with other re2c features, they are
    a bit broken and I'm pretty sure some users use multiple blocks
    (e.g. I used to do it).

    Thus we cannot just make '-f' generate state dispatch on per-block
    basis (there're some other obstacles: it's not quite clear which
    block '/*!getstate:re2c*/ directive is related to, etc.).

    I leave the situation 'as is' until better times (when lexer-
    parser loop is fixed and the whole code generation model is more
    robust).