Add daemon with multi-user support

[daonb]

To better support a shared dev server, we need a daemon that launches on boot and run as root.

Upon initialization, the daemon will scan all users $HOME looking
for ~/.config/webexec if found, it will read the config to get the user's
fingerprint (aka public key) and add it to its users list.
It will use the users list for two things:

• Accepting WHIP connection for all the users' fingerprints
• Update peerbook with the list of users' fingerprints

Upon a SIGHUP signal to the daemon it willclear the peer list and starts the scan.

Once a connection request is received a webexec user agent will be launched and used to establish the peer connection.

[daonb]

The daemon will also need to distribute the udp ports among the agents.

We need to document the config:
what stays in the ~/.config/webexec/webexec.conf
and what goes to ~/etc/webexec.conf