Dispatch post-activation messages to anyone interested

Dispatch post-activation messages to anyone interested

Previously, if a method call activated a service, it would only be
delivered to that service, and not to other services with match rules
which should match. This patch replaces the improperly-duplicated
dispatch code in activation.c with a call back into the normal dispatch
code, fixing this bug (fd.o#26427).

(Additionally, were one to service-activate a service that doesn't
understand file descriptors with a message containing a file descriptor,
the previous code would send it anyway, and the service's dbus library
would blow up. This is also fixed here, since the normal dispatch code
checks this correctly.)