unxz: update from XZ embedded git

unxz: update from XZ embedded git

function                                             old     new   delta
rc_reset                                               -      21     +21
unpack_xz_stream                                    2342    2357     +15
lzma_reset                                           102      64     -38
lzma_len                                             506     443     -63
xz_dec_lzma2_run                                    1438    1374     -64
xz_dec_reset                                          73       -     -73
lzma_main                                           2517    2183    -334
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 1/4 up/down: 36/-572)          Total: -536 bytes

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