MINOR: buf: Add function to realign a buffer with a specific head position

MINOR: buf: Add function to realign a buffer with a specific head position

b_slow_realign() function may be used to realign a buffer with a given
amount of output data, eventually 0. In such case, the head is set to
0. This function is not designed to be used with input only buffers, like
those used in the muxes. It is the purpose of b_slow_realign_ofs()
function. It does almost the same, realign a buffer. But it do so by setting
the buffer head to a specific offset.