discover/discover-server: Restrict clients based on uid

discover/discover-server: Restrict clients based on uid

If crypt support is enabled restrict what actions clients can perform by
default. Initial authorisation is set at connection time; clients
running as root are unrestricted, anything else runs as restricted until
it makes an authentication to pb-discover.

Unprivileged clients may only perform the following actions:
- Boot the default boot option.
- Cancel the autoboot timeout.
- Make an authentication request.

If a group named "petitgroup" exists then the socket permissions are
also modified so that only clients running as root or in that group may
connect to the socket.
The user-event socket is only usable by root since the two main
usecases are by utilities called by pb-discover or by a user in the
shell who will need to su to root anyway.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>