net: buf: Add net_buf_reserve() API

This API allows initializing the buffer with a given headroom. This
prepares the way for eventually removing the 'reserve_head' parameter
from the net_buf_get APIs, but can already now be useful in some
scenarios where the headroom is desired to be different than some
higher level API sets it to be.

Change-Id: Iffbe5761fdf3d2ad8cb4b8437b1074cf42ea9c6c
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>