support multiple prefixes and altering them during runtime

[christf]

This will allow l3roamd

• [x] be started without -b and without -i flags
• [x] to handle multiple prefixes
• [x] set l3roam0 interface "up"
• [x] set the route towards the l3roam0 interface based on the current prefixes
• [x] to allow adjustments of prefixes during runtime via socket.

and rework parameter handling for mandatory / optional parameters.

[christf]

@t-8ch I would appreciate your comments on that PR.

[christf]

... I will stop pushing unrelated changes to this PR now...

[christf]

when started without -i, l3roamd crashes. => done.

[christf]

there is a lot of stuff in here. merging now to get l3roamd back into a working state wrt NS and NA. For the rest we'll have to cope.