iconv: outsource code into it's own file

iconv: outsource code into it's own file

While testing, I found two already existing bugs:
* the variable 'a' passed to iconv_convert() needs to be passed by
  reference in order to allow for the desired side effect.
* Somehow the trailing junk after an iconv_conversion to a shorter
  string messes things up (and gets printed!). I couldn't exactly find
  out why this happens, but setting (*p) = 0; solves this problem.