Introduce btr_pool_get_buffers().

Introduce btr_pool_get_buffers().

This allows the users of the buffer pool API to obtain references to both parts
of the buffer pool area in case there is free space available at the end of the
area as well as at the beginning.

This is needed for reading from file descriptors where reading less than a full
datagram would discard the remaining part (DCCP and UDP).