Fix 'buffer_recv' to accept flags

Currently, 'buffer_recv' uses hard-coded flags containing 'MSG_DONTWAIT'
and functions that accept flags and invoke 'buffer_recv' do not actually
pass flags to it. Fix this so that 'buffer_recv' accepts flags.

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Use 'MSG_DONTWAIT' on FIX and FAST receive paths

As 'buffer_recv' no longer uses 'MSG_DONTWAIT' by default, explicitly
set it on the FIX and FAST receive paths.

Suggested-by: Pekka Enberg <penberg@iki.fi>
Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #24 from jvirtanen/fixes/buffer-recv-flags

Fix 'buffer_recv' to accept flags