*: teach tar et. al. to understand .xz by heart

*: teach tar et. al. to understand .xz by heart

function                                             old     new   delta
unpack_xz_stream                                       -    4126   +4126
setup_unzip_on_fd                                     80     150     +70
open_zipped                                          113     131     +18
unpack_unxz                                            5      12      +7
send_tree                                            360     353      -7
unpack_xz_stream_stdin                              3953       -   -3953
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 3/1 up/down: 4221/-3960)        Total: 261 bytes

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