base-files: fix rc.common help alignment

base-files: fix rc.common help alignment

This commit introduces a new function `extra_command` to better format
the help text without having to calculate the indentation in every startup
script that wants to add a new command. So far it looks weird and is not
formatted correctly on some startup scripts.

After using the new `extra_command` wrapper the alignement looks correctly.

And if the indentation is not sufficient in the future, this can be
changed in the function extra_command at a central location.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>