core: reinstate propagation of stop/restart jobs via RequsiteOf dependencies

core: reinstate propagation of stop/restart jobs via RequsiteOf dependencies

This reverts the primary effect of be7d9ff730cb88d7c6a869dd5c47754c78ceaef2.

After all Requisite= should be close to Requires=, without the one
exception that it doesn't pull in dependencies on start. However,
reverse deps on stop/restart should be treated the same way as for
Restart=, and this is already documented in the man page, hence stick to
it.

http://lists.freedesktop.org/archives/systemd-devel/2015-May/032049.html