community/s6-overlay: a brand new, maintained version

s6-overlay has been entirely rewritten and can now be maintained again. The MR does the following:

• add a symlink in the execline package, so #!/usr/bin/execlineb shebangs work
• package s6-overlay-helpers, a small binary package needed for s6-overlay
• move s6-overlay from unmaintained to community and make it work with the latest version (3.1.0.1)