emacs: Use make-process when available

emacs: Use make-process when available

make-process is a new function introduced in Emacs 25, which provides
greater control over process creation. Crucially, it allows
separately redirecting stderr directly to a buffer, which allows us to
avoid needing to use the shell to redirect to a temporary file in
order to correctly distinguish stdout and stderr.

* notmuch-lib.el: Use make-process when it is available; fall back to
  the previous method when not.