archival: add option -k "keep" to gzip/bzip2/lzop, add -U "nokeep" to lzop

archival: add option -k "keep" to gzip/bzip2/lzop, add -U "nokeep" to lzop

function                                             old     new   delta
bbunpack                                             745     779     +34
lzop_main                                             93     121     +28
do_lzo_compress                                      320     328      +8
packed_usage                                       31685   31653     -32
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/1 up/down: 70/-32)             Total: 38 bytes

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