Action option: always run helper scripts, even if the device exists

[jcnelson]

In some early boot configurations, a device node can be created by some other process besides vdev. However, sometimes vdev should still run its helper scripts. To remedy this, we should add "if_exists=" to the action ini format that takes a comma-separated list of options:

• run_helpers: run the helper scripts even if the device exists
• no_error: do nothing, but don't report -EEXIST

TODO: add more options to if_exists as we need them, based on user feedback.

[jcnelson]

Added as of 71923cab3703048349433dd4a58b5432d776fe43.