router: add option to include custom routes in RA

[dbnk0]

Introduces new configuration option ra_staticroute for adding custom route(s) in route advertisement (RA) messages when no default route is advertised.

This is useful when, for example, end devices are given only ULA IPv6 addresses (e.g. from fc00:0:100::/48), but one still needs inter subnet communication to e.g. fc00:0:200::/48. Currently, this wouldn't be possible without static route on each end device as the odhcpd server doesn't present itself as default router and no route information is given except for local ULA subnet (fc00:0:100::/48).

New configuration option ra_staticroute makes it possible to advertise arbitrary routes to the end devices and thus making correct routing possible even when no default route (or public IPv6 prefix) is available.

Option ra_staticroute exists under both odhcpd section and interfaces' sections. Latter sets up interface-specific routes. Routes configured under odhcpd are global and advertised for all interfaces (including those with interface-specific static routes).

Config example:

config odhcpd 'odhcpd'
	...
	list ra_staticroute 'fc00:10::/32'
	list ra_staticroute 'fc00:20::/32'

config dhcp 'lan'
	...
	list ra_staticroute 'fd50:60:70::/48'

Fixes: #74 and possibly helps with #152

[systemcrash]

I think this is a good option to have.

@DavidB137 please fix the conflicts.

[dbnk0]

Sorry, I messed up quite a bit when resolving the conflicts. At first, I used spaces instead of tabs for indentation and then tried to fix it (unsuccessfully). So I instead synced my fork with upstream and reapplied the changes which should also make the history cleaner as there's no merge commit. Once again, sorry for the confusion.

@systemcrash Hopefully you can merge now.

[systemcrash]

I wonder whether it'd be better to call them list ra_customroute or list ra_staticroute instead? Add is a verb which might suggest something else than what they are for. What do you think @dbnk0 ?

[dbnk0]

I completely agree with you, list ra_staticroute seems like the best option to me.

I renamed everything, did some refactoring, removed unnecessary dynamic memory allocation and improved error handling.
In case you want to review rename and everything else individually, these are the commits: f1ee9c6, 4fec3c8, 8b8b1cc, af20724.

I squashed everything together now.

@systemcrash Let me know if you want to change anything else.

[systemcrash]

This still looks great - more readable now. Could you update your initial post example please?

[dbnk0]

Updated