bfd: Multiple buffers management (v2)

I plan to re-use the bfd engine for images buffering. Right
now this engine uses one buffer that gets reused by all
bfdopen()-s. This works for current usage (one-by-pne proc
files access), but for images we'll need more buffers.

So this patch just puts buffers in a list and organizes a
stupid R-R with refill on it.

v2:
  Check for buffer allocation errors
  Print buffer mem pointer in debug

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>