Support "system" properties, i.e., library versions, whether
language-detection, scm is supported.
Make these more generic (with display strings), so we can handle them
more automatic int mu-cmd-info.
This makes things a bit more uniform with all the places where we use
"personal".
--my-address remains as an alias for the command-line option.
Add unit test.
Resolves#2806.
Xapian supports an "ngrams" option to help with languages/scripts
without explicit wordbreaks, such as Chinese / Japanese / Korean.
Add some plumbing for supporting this in mu as well. Experimental for
now.
