Add a facility for sending synthetic udev events

[mulkieran]

The user running these will need root permissions. The value this adds is that the user need not know the details of the "/sys" directory.

[mulkieran]

Related to stratis-storage/project#50 since sending synthetic udev events might be what we need to trigger discovery of new device size.

[mulkieran]

We discussed this in meeting and some points emerged.

1. We should put it in the top-level debug sub-menu.
2. We need a command name, e.g., uevent. The arguments to the command are just the devnode and the event type: change, add, and remove.

The benefit of the code will be that it will sort out where in sysfs to find the file to write to, so that the user doesn't have to.

[mulkieran]

Tentatively re-assigning.