tar: skip leading / and handle names like abc/..////def -> def (not ///def)

tar: skip leading / and handle names like abc/..////def -> def (not ///def)

function                                             old     new   delta
strip_unsafe_prefix                                    -     105    +105
writeFileToTarball                                   557     520     -37
get_header_tar                                      1545    1462     -83

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>