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
update TODO
dbus: Do send out "replies" to signals

Some parts of systemd (at least the DBus activation codepath) "reply"
to signals, which of course have the no-reply flag set.  We will be
defensive here and still send out a reply if we're passed a signal.

Regression introduced by: c6a818c82035da91e

Reported-by: Mantas Mikulėnas <grawity@gmail.com>
Tested-by: Mantas Mikulėnas <grawity@gmail.com>