block: generate hotplug.d mount events

block: generate hotplug.d mount events

With this change block generates 2 "mount" hotplug.d subsystem events:
1) "add" when block device gets mounted
2) "remove" when block device gets unmounted

This allows e.g. controlling USB storage dependant software using
hotplug.d listeners.

A very similar solution was implemented in mountd which was replaced by
blockd.

Right now this is implemented using a call to the /sbin/hotplug-call.
A possible improvement is to rewrite above shell script into a C lib
function. For now let's just assume that script exists.

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