CLI: simplified handling of separated-fields arguments

CLI: simplified handling of separated-fields arguments

this patch reduces complexity associated with
commands requiring a separated arguments
such as :
-o filename
-D dictionary
--output-dir-flat dir
--output-dir-mirror dir

It used to be a multi-stage process with explicit context,
it's now simplified as a single macro.

Thanks to this simplification,
separated arguments logic has also been extended to
--patch-from XXX
--filelist XXX
which extends existing capability using =XXX
--patch-from=XXX
--filelist=XXX

Separated argument is useful for filenames and directories,
as it benefits from shell expansion
such as ~/dir/file
where the ~ is automatically translated by the shell.
In contrast --long-command=FILE does not interpret FILE,
so ~/ is transmitted as is to the main() function,
generally resulting in incorrect file name.