Improve mbuf allocation behavior

 * Limit total amount of headroom, in absolute terms (`MBUF_SIZE_MAX_HEADROOM`).
 * If unable to allocate with headroom, fall back to allocating the required minimum.
 * For mOS, set default `MBUF_SIZE_MULTIPLIER` to 2 to avoid floating point operations.
   Since max headroom size is now capped to 128 bytes, this will not result in much of a bloat.

PUBLISHED_FROM=11d4fc65a46a805bb7c8960f89a3d0b753c58bb8