node: Don't ignore return value of setgid / setuid

* With activated hardening flags -Werror=unused-result breaks the build if
  return values are ignored
Remove not longer needed patch from debian/patches

PLUGINDIR is now correct set in the Makefile
Don't use debconf. Simply Conflict: munin-node

* For now we don't ask the user if munin-node-c or munin-node should used.
  Simply conflict munin-node and start munin-node-c
Remove not longer needed debian/TODO
Add get-orig-source target to debian/rules