Add support for extensions.conf

Starting with the [general] section, add support for the extensions.conf
file. Because we use concat, an integrator can always add
concat::fragment resources to append or prepend info to this file.

Additionally, cleaned up the path references for $astetcdir.
Extend the user.pp to include more options

Added was the `read`, `write`, `permit`, `deny`, and `eventfilter`
options. Moved the content to an ERB template.
Merge pull request #11 from runswithd6s/extend-user

Extend the user.pp to include more options
Merge pull request #10 from runswithd6s/extensions

Add support for extensions.conf