block(d): improve hotplug.d "mount" events for the autofs

block(d): improve hotplug.d "mount" events for the autofs

So far - when using autofs - "add" and "remove" action events were
triggered on every (un)mount. It wasn't very helpful when using autofs
due to its nature.

The point of blockd & autofs is to have block devices mounted on an
access request. Its users should not care / know if it's currently
mounted or not. Mounting should be handled transparently.

To make that work it requires informing listeners whenever device:
1) Becomes *ready* for mounting (by triggering "add" action)
2) Becomes *unavailable* (by triggering "remove" action)

The current mounting state is something that autofs & blockd should
handle internally and should not notify listeners about.

This is implemented by:
1) block generating events for non-autofs cases only (when (un)mounting)
2) blockd informing block when autofs resource becomes (un)available

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>