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).

Disabling the piggyback of timestamps can be useful if the per-message
overhead is not desirable, or for troubleshooting compatibility issues
(there should be none, but who knows).

Note that this commit is a no-op, since this condition is true by
default, and it cannot be set in the configuration yet.

Note that this commit effectively disables the computation of a cost
from the RTT, since 'max-rtt-penalty' defaults to 0.

To do this, we use the timestamps piggybacked on Hello and IHU
messages.

We use a sub-TLV to do so, which we fill out at the very last moment
to get an accurate timing.

This timestamp will be used to compute the RTT to our neighbours.

Note that babeld's behaviour is not modified by this commit.

This will be used to embed a timestamp into Hello messages, which will
eventually allow to compute the RTT between nodes.

parse_msec() is not specific to milliseconds, it can parse any
fixed-point string.  It can be used, for instance, to parse a
fixed-point string written in milliseconds and interpret it as an
integer in microseconds.