[backend] bs_signer: don't serialize before checking exit/restart

also support $BSConfig::signer_maxchild
[backend] Pass vm-disk option down to build

Make sure that build uses the same
vm-disk than bs_worker in the emulator backend case.
[backend] strip flags from remote meta, make getpackagelist die on missing remote projects
[webui] There is no reason to hide our awesome users
[api] Get rid of the registration notes