This is a MUST according to the Babel Extension Mechanism draft
(Section 2.2.3).

This reverts commit bf8555b2.

This is consistent with Baptiste's work, and is better in any case.

Makes it consistent with the other interface flags.

Without this, later interface definitions override earlier ones
even when they don't specify a value.

Fixes compilation for MIPS.

Conflicts:
	CHANGES

We were currently rounding down the result, which could cause the
smoothed RTT to get permanently stuck below the RTT samples.

We now round either up or down, whichever gets us closer to the
current RTT sample.

Note that this slightly increases the amount of information sent out
to the local interface (up to three neighbour updates per received
packet, vs. up to two without this patch).