Allow setting a separate policy for the user and system buses.
For now, the filter policy is equivalent to the none (block) policy.
Future commits will add more configuration options and filters.

To contain processes forked for long time, such as the xdg-dbus-proxy,
sbox_exec_v can be used, which is the non-forking version of sbox_run_v.
Additionally, the SBOX_KEEPS_FDS flag avoid closing any open fds,
so fds needed by the subordinate process can be left open before calling
sbox_exec_v.
This flag does not makes sense for sbox_run_v, and causes an assertion failure.

* The proxy is forked off outside the sandbox namespace to protect the
  fds of the original buses from the sandboxed process.
* The /run/firejail/dbus directory (with the sticky bit set) holds the proxy
  sockets. The sockets are <parent pid>-user and <parent pid>-system for the
  user and system buses, respectively. Each socket is owned by the sandbox user.
* The sockets are bind-mounted over their expected locations and the
  /run/firejail/dbus directory is subsequently hidden from the sandbox.
* Upon sandbox exit, the xdg-dbus-proxy instance is terminated and the sockets
  are cleaned up.
* Filter rules will be added in a future commit.

The options --dbus-user.talk, --dbus-user.own, --dbus-system.talk, and
--dbus-system.own control which names can be accessed and owned on the user and
system buses.

To avoid race conditions, the proxy sockets from /run/firejail/dbus/ are
bind-mounted to /run/firejail/mnt/dbus/, which is controlled by root.

Instead of relying on the default locations of the DBus sockets, the environment
variables DBUS_SESSION_BUS_ADDRESS and DBUS_SYSTEM_BUS_ADDRESS are set
accordingly.

User sockets are tried in the following order when starting the proxy:
* DBUS_SESSION_BUS_ADDRES
* /run/user/<pid>/bus
* /run/user/<pid>/dbus/user_bus_socket
These are all blocked (including DBUS_SESSION_BUS_ADDRESS if it points at a
socket in the filesystem) when the filtering or blocking policy is active.

System sockets are tried in the following order:
* DBUS_SYSTEM_BUS_ADDRESS
* /run/dbus/system_bus_socket
These are all blocked (including DBUS_SYSTEM_BUS_ADDRESS if it points at a
socket in the filesystem) when the filtering or blocking policy is active.

This patch also allows setting the DBus policies to filter even if
xdg-dbus-proxy is not installed. In that case, unrestricted access to the bus is
allowed, but a warning is emitted.