registerurl,unregisterurl: rework output and support --json

registerurl,unregisterurl: rework output and support --json

* registerurl, unregisterurl: Improved output when reading from stdin
  to be more like other batch commands.
* registerurl, unregisterurl: Added --json and --json-error-messages options.

Note that this did change the --batch output in a way that could possibly
break something that expected the old output to never change. I think it's
acceptable to break that because there has never been a guarantee of
unchanging output format except with --batch for most commands. The old
output was just really weird too!

One possible wart is that "git-annex registerurl" with no options now
seems to just hang, since it's waiting for stdin input. Before, it said
"registerurl (stdin)" which was clearer about what's happenening. But this
is a deprecated mode anyway, --batch makes clear what's happening. If
anything, this problem would be a reason to eventually remove the support
for reading from stdin w/o --batch.

Sponsored-by: Dartmouth College's Datalad project