Per-user, unprivileged device rules actions

[jcnelson]

Let users define their own device rules and actions (let's say under ~/.vdev/ or the like).

• vdevd will run an action if the user is authorized to define actions (under /etc/vdev/users.list or the like)
• The action will run under the user's UID and the "plugdev" GID (replace "plugdev" with a configurable, potentially per-user GID).
• vdevd will only run the action if the device file is visible to the user (i.e. according to the ACLs)

The motivation is to let unprivileged applications (i.e. desktop environments) specify actions to take when the user plugs something in, or a device comes online.

[jcnelson]

The way we'll do this is create a second vdevd backend--one that watches the filesystem instead of listens for new devices from the kernel. A user can start it, or it can be started as part of the .bashrc. It will behave exactly like the "root" vdevd insofar as setting it up (actions and scripts). Critically, it (1) will only be able to access devices the user is allowed to (or even see, with vdevfs), and (2) its actions will run with user privileges, not root privileges.