new variable 'unlink'

Solve issue #11 by allowing the use of --unlink wget option

Use the following line in mirror.list to enable it:
 set unlink 1
delete index file before writing to it

Otherwise, the file is writen in place and changes the hardlinked
ones too in older trees, therefore introduces inconsistencies in
older snapshotted trees.
Merge branch 'mirror_to_hardlinked_directory' into master-dr4ke

Conflicts:
	apt-mirror
	mirror.list