MINOR: buffer: add bo_del() to delete a number of characters from output

MINOR: buffer: add bo_del() to delete a number of characters from output

This simply reduces the amount of output data from the buffer after
they have been transferred, in a way that is more natural than by
fiddling with buf->o. b_del() was renamed to bi_del() to avoid any
ambiguity (it's not yet used).