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

* With activated hardening flags -Werror=unused-result breaks the build if
  return values are ignored
Merge remote-tracking branch 'github-pull/17'

Merge fix for build with -Werror=unused-result, thanks to Matthias
Schmitz.
need to mentions *.pod files in EXTRA_DIST

Otherwise they are not included in "make dist" and compilation fails
with:

make[1]: *** No rule to make target `munin-node-c.1', needed by `all-am'.  Stop.