Wires up systemd support to configure DNS on startup and when
enabling/disabling exit mode.
On startup (and when turning off an exit) we tell systemd-resolved to
direct .loki and .snode lookups to lokinet (leaving other DNS traffic
alone).
On exit enabling, we reconfigure it to resolve "." (i.e. the root DNS
domain) so that all lookups come into it.
All #ifndef guards on headers have been removed, I think,
in favor of #pragma once
Headers are now included as `#include "filename"` if the included file
resides in the same directory as the file including it, or any
subdirectory therein. Otherwise they are included as
`#include <project/top/dir/relative/path/filename>`
The above does not include system/os headers.
If not using an exit, messing with the routing table is not good.
As such, the ability to keep track of routes we *might* want is good,
but the ability to set/unset those routes is necessary, to correspond
to enabling/disabling exit functionality.
* * refactor route poking out of llarp::Router and into llarp::RoutePoker
* only poke routes when we have an exit enabled as a client
* add route_poker header so it compiles
