We only have two real DNS plugins: "dnsmasq" and "systemd-resolved" (the "unbound"
plugin is very incomplete and should eventually be dropped).

Of these two, only "dnsmasq" spawns a child process. A lot of the logic
for that is in the parent class NMDnsPlugin, with the purpose for that
logic to be reusable.

However:

 - We are unlikely to add more DNS plugins. Especially because
   systemd-resolved seems the way forward.

 - If we happen to add more plugins, then probably NetworkManager
   should not spawn the process itself. That causes problems with
   restarting the service. Rather, we should let the service manager
   handle the lifetime of such "child" processes. Aside separating
   the lifetime of the DNS plugin process from NetworkManager's,
   this also would allow to sandbox NetworkManager and the DNS plugin
   differently. Currently, NetworkManager itself may might need
   capabilities only to pass them on to the DNS plugin, or (more likely)
   NetworkManager would want to drop additional capabilities for the
   DNS plugin (which we would rather not implement ourself, since that
   seems job of the service management already).

 - The current implementation is far from beautiful. For example,
   it does synchronous (blocking) killing of the running process
   from the PID file, and it uses PID fils. This is not something
   we would want to reuse for other plugins. Also, note that
   dnsmasq already spawns the service asynchronosly (of course).
   Hence, we should also kill it asynchronously, but that is complicated
   by having the logic separated in two different classes while
   providing an abstract API between the two.

Move the code to NMDnsDnsmasq. This is the only place that cares about
this. Also, that makes it actually clearer what is happening, by seeing
the lifetime handling of the child proceess all in one place.