cylc unregister: skip suites with port files

[matthewrmshin]

Also ensure that cylc unregister bar does not unregister a suite
called foobar.

Close #1796.

[matthewrmshin]

@hjoliver @arjclark please review.

[hjoliver]

Branch needs updating.

[hjoliver]

This block (in master too) is a bit too simplistic to do what it's supposed to - i.e. remove all empty directories left by unregistering and deleting a bunch of suites. To see this, try cylc import examples $TMPDIR then cylc unreg -d cylc-.* and look in $TMPDIR. However, I doubt that anyone stores suites in directory trees that reflect the suite name hierarchy as I originally intended (e.g. nwp.test1 and nwp.test2 stored in nwp/test1/ and nwp/test2) - certainly Rose users don't - and unregisters them en-masse ... so maybe we should just remove this feature.

[matthewrmshin]

All comments addressed. Branch re-based.

[hjoliver]

Review 1 - good, test battery passing.

[arjclark]

Looks OK to me. No problems from the test-battery in my environment.