L::Command: Add support for a calling a sub in the child

Signed-off-by: Niels Thykier <niels@thykier.net>
c/po-debconf.pm: Use L::Command to avoid spawning a shell

Signed-off-by: Niels Thykier <niels@thykier.net>
t: Bump ancient date

Signed-off-by: Niels Thykier <niels@thykier.net>