Allow sorting developer signatures when creating mbox

[glaubitz]

When importing patches from a mailing list thread, b4 adds the developer signatures it finds in the thread. While that works very well, the sorting order of the signatures in the commit message the patches seems rather non-deterministic.

In order to increase readability, it would be nice if there was an option to sort the signatures by developer name while keeping the Signed-off-by of the author at the top and the Signed-off-by of the subsystem maintainer at the bottom.

[mricon]

The signed-off-by order is deterministic and follows the chain-of-custody rules. See the documentation for "trailer-order": https://b4.docs.kernel.org/en/latest/config.html#core-options