configure: move make {,install} instructions to the end

There was theorical possibility that writing the config files could
have skipped (by interruption) after the instructions how to make
notmuch was printed out.
cli: add support for notmuch search --duplicate=N with --output=messages

Print the message IDs of all messages matching the search terms that
have at least N files associated with them.
test: add tests for --output=messages --duplicate=N

Basic smoke tests for the feature, nothing fancy.
test: Make gen-threads work with python3

python3 doesn't allow dictionaries to be initialized with non-string
keywords. This presents problems on systems in which "python" means
"python3". We instead initalize the dictionary using the dict
comprehension and then update it with the values from the tree. This
will work with both python2 and python3.
devel: make man-to-mdwn.pl to work with generated manual pages

The new manual pages converted from rst using sphinx or rst2man
has somewhat different syntax. man-to-mdwn.pl is now adjusted
to produce even better output from this syntax. The changes also
include using utf-8 locale (e.g. for tables and generated hypens)
and and quite a few bugs fixes.
This tool still produces better results than just using the
html pages generated using sphinx / rst2html. For example those
tools don't create inter-page hyperlinks -- and the preformatted
pages written by man-to-mdwn.pl just works well with manual page
content.
devel: man-to-mdwn.pl: add GPLv3+ notice to manpages.mdwn

Make the generated head manual page (in the notmuchmail wiki)
display the licence information the whole notmuch software is licenced
under.