fix jpegtran metadata stripping

The option «-copy» of jpegtran is used in the wrong way.

If stripping metadata is turned on the argument gets the value «all» otherwise «none».
Jpegtran copies the metadata if «all», «none» does omit them.

I renamed the variable «strip-meta-tags» to «retain-meta-tags», that should be more clear

    $ jpegtran --help
    usage: jpegtran [switches] [inputfile]
    Switches (names may be abbreviated):
      -copy none     Copy no extra markers from source file
      -copy comments Copy only comment markers (default)
      -copy all      Copy all extra markers