Switch to imsg for privsep communication

[sspans]

Currently there are two socketpairs, one used for requests and one for messages. These can be merged and switched over to openbsd's imsg.

This will require major refactoring of the ladvd internals, which is why I'd like to do it. The goal is to make all messaging fully async.

[sharewax]

Hello, I suppose, that 1.1 will be released in few weeks? :)