NetworkPluginServer: handle argc/argv sanely when spawning backends

- The old code did some really silly things when spawning backends -
  on the level of "make the arguments into one string and then parse" (!)
- We now actually use std::vector and do this the competent way.
- Plus, no more dependency on popt, which I can't believe was ever
  included...!
- execvp() instead of execv() is used, so you don't have to specify the
  full PATH (useful for nixos)