Merge lp://staging/~mterry/mediascanner2/only-in-u8 into lp://staging/mediascanner2

Only run mediascanner in unity8.

As part of the MIR for mediascanner2 (bug 1669546), it was noted that mediascanner2 caused some issues with unity7 (scanning thumb drives when the user is trying to eject and some such). So to avoid that for now and to get mediascanner2 into main for zesty, here's a patch to only run mediascanner2 in unity8.

I've chosen to use a runtime function to check the environment because (A) the environment checking is not trivial and (B) systemd units don't seem to be as easy to say "only run in unity7" as upstart jobs.

I've created the new environment env MEDIASCANNER_RUN=1 if someone would want to manually override this check.

I've only guarded the main scanner daemon, since that's the bit that is unconditionally run and likely to conflict with thumb drives. The extractor and dbus interface both seem harmless (and only activated upon request). Is that accurate?

I didn't add any tests, since none of the current tests try running the daemon itself. But I manually tested.