unxz: new applet, complete with xzcat and xz -d aliases

unxz: new applet, complete with xzcat and xz -d aliases

function                                             old     new   delta
unpack_xz_stream_stdin                                 -    3953   +3953
lzma_main                                              -    2601   +2601
lzma_len                                               -     516    +516
dec_vli                                                -     165    +165
dict_repeat                                            -     103    +103
lzma_reset                                             -      98     +98
fill_temp                                              -      98     +98
crc32_validate                                         -      93     +93
xz_dec_reset                                           -      77     +77
unxz_main                                              -      77     +77
index_update                                           -      47     +47
xz_crc32                                               -      40     +40
packed_usage                                       27044   27060     +16
make_new_name_unxz                                     -      14     +14
applet_names                                        2240    2254     +14
applet_main                                         1312    1324     +12
applet_nameofs                                       656     662      +6
unpack_unxz                                            -       5      +5
send_tree                                            355     360      +5
applet_install_loc                                   164     166      +2
------------------------------------------------------------------------------
(add/remove: 15/0 grow/shrink: 6/0 up/down: 7942/0)          Total: 7942 bytes
   text    data     bss     dec     hex filename
 844032     453    6812  851297   cfd61 busybox_old
 852063     453    6812  859328   d1cc0 busybox_unstripped

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