dd: add 'oflag=seek_bytes'

dd: add 'oflag=seek_bytes'

Allow specifying position in the output file in bytes instead of
obs-sized blocks, improve compatibility with GNU dd.

function                                             old     new   delta
dd_main                                             1632    1693     +61
packed_usage                                       33130   33150     +20
static.oflag_words                                     -      12     +12
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 2/0 up/down: 93/0)               Total: 93 bytes

Signed-off-by: Rostislav Skudnov <rostislav@tuxera.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>