exec: Assigning the empty string to CapabilityBoundSet= should drop all caps

exec: Assigning the empty string to CapabilityBoundSet= should drop all caps

Previously, it would set all caps, but it should drop them all, anything
else makes little sense.

Also, document that this works as it does, and what to do in order to
assign all caps to the bounding set.

https://bugzilla.redhat.com/show_bug.cgi?id=914705