[unit] deref symlinks on restoration

Otherwise symlinks pointing to nowhere will appear in destination
instead of files. Also to avoid data relacement failure when destination
contains symlinks etc., first destination is removed if mode is
"overwrite"

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
Merge pull request #3 from deztructor/master

[unit] deref symlinks on restoration