Allow in-place extraction of a tar archive

Allow in-place extraction of a tar archive

* scripts/Dpkg/Source/Archive.pm: New "in_place" option to
extract an archive in the target directory instead of replacing
the whole target directory. Replace the logic to fix permissions
by fixperms() which is a...
* scripts/Dpkg/Source/Functions.pm (fixperms): new function to
fix permissions on a given directory so that they match the rights
expected through the umask.