units: express Conflict in syslog.socket instead of emergency.service

Note this commit only changes how the code is expressed; it does not change
the existence of any dependency.

The `Conflicts=` was added in 3136ec90, "Stop syslog.socket when entering
emergency mode".  The discussion in the issue #266 raised concerns that
this might be needed for other units, but failed to point out why
syslog.socket is special.  The reason is that syslog.socket has
DefaultDepedencies=no, so it does not get Requires=sysinit.target like
other socket units do.  But syslog.service does require sysinit.target,
among other things.

We don't have many socket, path, or timer units with
DefaultDependencies=no, and I don't think any of the triggered services
have such additional hard dependencies as syslog.service does.

It is much less confusing if we keep this `Conflicts=` in the same file as
the `DefaultDependencies=no` which made it necessary.