Workaround for a problem of irreversible filename conversions

[devgs]

Annotation:

Sometimes there might be a problem converting UTF-8 to zips native
charset(CP866), because it's not 1:1 conversion. So my solution is to
allow
developers provide their own version of converted filename and pass it
to mod_zip along with UTF-8 filename which will go straight to Unicode
path extra field (thanks to tony2001). So separator is a solution that
doesn't
break current format. And allows passing file name in both formats as
one string.

Normally we pass:
CRC32 [size] [path] [filename]\n
...

• [filename] passed to archive as filename w/o conversion

• UFT-8 flag for filename is set

  tony2001's X-Archive-Charset: [charset] way:
  CRC32 [size] [path] [filename]\n
  ...

• [filename] is accepted to be UTF-8 string

• [filename], converted to [charset] and passed to archive as filename

• [filename] passed to Unicode path extra field

• UFT-8 flag for filename is not set

  My X-Archive-Name-Sep: [sep] solution:
  CRC32 [size] [path] [native-filename][sep][utf8-filename]\n
  ...

• [native-filename] passed to archive as filename w/o conversion

• [utf8-filename] passed to Unicode path extra field

• UFT-8 flag for filename is not set

  You just need to provide separator that won't interfere with file
  names. I suggest using '/'
  as it is ASCII character and forbidden on most (if not all) platforms
  as a part of filename.

  Empty separator string means no UTF-8 version provided. Usefull when we
  need to pass only
  names encoded in native charset. It's equal to 'X-Archive-Charset:
  native;'.
  Note: Currently it is impossible after '[PATCH] Support for UTF-8 file
  names.'(4f61592)
  because UFT-8 flag (zip_utf8_flag) is set default for templates.