emacs: Improve the definition and use of `notmuch-fcc-dirs'.

emacs: Improve the definition and use of `notmuch-fcc-dirs'.

Re-work the declaration and definition of `notmuch-fcc-dirs'. The
variable now allows three types of values:

- nil: no Fcc header is added,

- a string: the value of `notmuch-fcc-dirs' is the name of the
  folder to use,

- a list: the folder is chosen based on the From address of the
  current message using a list of regular expressions and
  corresponding folders:

     ((\"Sebastian@SSpaeth.de\" . \"privat\")
      (\"spaetz@sspaeth.de\" . \"OUTBOX.OSS\")
      (\".*\" . \"defaultinbox\"))

  If none of the regular expressions match the From address, no
  Fcc header will be added.