Move daemon and stub to /usr/libexec

[cgwalters]

I was spending some time investigating how update_engine works, and when first
starting out it took me a little bit to figure out that there were separate
update_engine_client vs update_engine. Having them both in /usr/sbin is
annoying for bash completion.

This is just a minor "papercut"; I'm mostly using this to test running
through the development/contribution process.

I am assuming nothing tries to find these binaries directly and only uses the
systemd service and/or DBus API; I didn't see any obvious hits in repo grep update_engine.

Signed-off-by: Colin Walters walters@verbum.org

[ajeddeloh]

LGTM, but I worry if this will break anyone who has systemd dropins for update-engine. If they have a dropin overwriting the ExecStart to add additional flags, this will break them. I'm not sure if there's actually users doing that, but it's something to consider.

[cgwalters]

Do you (or anyone) have a sense of whether there are flags that people might want to tweak? update_engine --help doesn't initially look to me like there are any outside of a development scenario but I'm obviously new here 😄

[lucab]

While I think that this papercut is technically ok, I think that at this point in CL lifetime it is not worth pursuing this with the risk of breaking existing customized setups. I'd be inclined to just close this PR.