Normalize file names to make sure they cannot traverse directories.

Some archive formats (tar only?) support relative file names,
that is, a file could be extracted at ../ instead of in ./.
This potentially allows files to "escape" from MComix extraction
directory, potentially overwriting other files the user running
MComix has write priviledges to.

Prevent this behavior by normalizing the extracted file name first.

git-svn-id: svn+ssh://svn.code.sf.net/p/mcomix/code@1040 ef8490bc-f317-4725-81ed-e95a9aca1bcf