Put --daemon in mrtg initfile (diff/t semantics than in conf)

 [as explained in the MRTG manpage]...

```
 --daemon
  Put MRTG into the background, running as a daemon. This works the same
  way as the config file option, but the switch is required for proper
  FHS operation (because /var/run is writable only by root)
```
...and the init file should *always* spawn mrtg as a daemon anyway if at
all possible (otherwise init needs to brute-force backgrounding it after
the fact, which is advised against in start-stop-daemon manpage on Debian
- and probably applies in other init systems too).
Put --pid-file in mrtg init daemonargs (always needed)

  Explicitly set mrtg flag --pid-file=$PIDFILE in init file because (being
  init) pid file is *always* needed, and is already hard-coded there for
  the stop() command. This avoids code-duplication in the config file, and
  consequent possibility for mismatches (resulting in non-killed mrtg
  processes). Set PIDFILE *before* referencing it (reorder) setting of
  vars.
Make setting and use of vars in init-file more DRY
Merge pull request #113 from rowanthorpe/improve_mrtg_conf

Improve mrtg init file