server: add CommandoSocket where multiple users can register commands

server: add CommandoSocket where multiple users can register commands

This is a preparatory step to replace the task control socket with it
and provide a "reopen log file" command for the rest server.

Kept it simple by disallowing to register new commands after the
socket gets spawned, this avoids the need for locking.

If we really need that we can always wrap it in a Arc<RWLock<..>> or
something like that, or even nicer, register at compile time.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>