Move common files to -common subpackage

The 'nginx' package as the 'base package' only caused confusion
(it didn't provide any real 'nginx' functionality) and a dependency loop
(which could end with broken install).

Also:
- subpackage dependencies fixed (e.g. they don't all need useradd, etc.)
- 'nginx should have own group' todo item removed (nginx can use both
  its own and the 'http' group)