emacs: prefer notmuch-emacs-version in User-Agent: header

Now that we have `notmuch-emacs-version' defined in notmuch emacs MUA
use that as a part of User-Agent: header to provide more accurate
version information when sending emails.

In case some incomplete installation of notmuch emacs MUA is used and
`notmuch-emacs-version' is defined as "unknown" then fall back to ask
version info from cli (as it used to be before this commit).

Requiring notmuch-version[.elc] and if that is missing setting
"fallback" notmuch-emacs-version (to "unknown") was moved from
notmuch.el to notmuch-lib.el as notmuch-mua.el (which provides
User-Agent: information) require's the latter.