notmuch: replace built-in help with exec of man

"notmuch help" now relies on a working man installation, and a correct
setting of MANPATH, for users that install in "unusual" places. This
should probably noted in NEWS before shipping.

There is still some small duplication of docs as the one line
summaries are hard coded in the binary. This seems preferable to
complicated script games, at least as long was we are using a
presentation level markup like nroff.