losetup: implement -c

losetup: implement -c

function                                             old     new   delta
losetup_main                                         422     449     +27
packed_usage                                       33243   33264     +21
get_next_block                                      1677    1681      +4
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 52/0)               Total: 52 bytes

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