Tools: Added subcommands to setup-unit.

This script combines the old setup-unit (as the repo-config command),
with new functionality, to provide an easy welcome website for
first-time users, and also some more commands that are useful for
administrating a running unitd(8) instance.

Suggested-by: Liam Crilly <liam@nginx.com>
Cc: Konstantin Pavlov <thresh@nginx.com>
Cc: Artem Konev <a.konev@f5.com>
Cc: Timo Start <t.stark@nginx.com>
Cc: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>