Finally removed auxiliary code generation pass to 'null device'.

Finally removed auxiliary code generation pass to 'null device'.

This pass was used to gather statistics:
    - which labels are used (to avoid 'unused label' warnings from
      C/C++ compiler)
    - in '-f' mode, how many times YYFILL is called (to generate
      dispatch to the arrpopriate YYFILL call and resume lexing from
      there)

This commit deals with second case: it makes counting YYFILL calls
independent of auxiliary pass. Counting relies on variable
're2c::last_fill_index', which is updated in 're2c::need' function
(note that it is crucial that 're2c::need' is always called prior
to generation of state dispatch).

Also fixed documentation for '-f'.