BUG/MEDIUM: ensure that unresolved arguments are freed exactly once

BUG/MEDIUM: ensure that unresolved arguments are freed exactly once

When passing arguments to ACLs and samples, some types are stored as
strings then resolved later after config parsing is done. Upon exit,
the arguments need to be freed only if the string was not resolved
yet. At the moment we can encounter double free during deinit()
because some arguments (eg: userlists) are freed once as their own
type and once as a string.

The solution consists in adding an "unresolved" flag to the args to
say whether the value is still held in the <str> part or is final.

This could be debugged thanks to a useful bug report from Sander Klein.