Rename parse_msec() and getmsec() to parse_thousands() and getthousands()

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.
Introduce accumulate_int and accumulate_unicast_int for 32-bits values
Introduce a pointer to the Hello message inside a node's send buffer

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.
Piggyback a timestamp on all outgoing Hello 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.
Keep track of timestamps on incoming Hello messages
Send back timestamps with outgoing IHU messages
Parse timestamps on incoming IHU messages
Compute the RTT to a neighbour

To do this, we use the timestamps piggybacked on Hello and IHU
messages.
Compute the moving exponential average of the RTT
Display the RTT of neighbours on the local interface
Add a cost to neighbours, computed from the RTT
Also display RTT cost on the local interface
Make 'rtt_exponential_decay' relative to an interface
Allow all RTT parameters to be configured

Note that this commit effectively disables the computation of a cost
from the RTT, since 'max-rtt-penalty' defaults to 0.
Add a condition for sending timestamps on Hello and IHU

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

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).
Display the RTT and associated cost on table dump
Update the metric of a neighbour's routes when its rttcost changes

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).
RTT smoothing: fix a rounding bug

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.
Print RTT values in milliseconds rather than microseconds