e155e0a976
Make sure to use the active default values for preamble and postamble in the help text,
and provide the current default values as the default in the add_argument().
The only exception is uccnc_post.py, where the real default value depend on the use of
--no-comments, and sending the default argument to add_argument() would change behaviour.
This change was inspired by the changes done to fix (#20792) in
d84d9c9bd1.