emacs: fix declare-function definitions with related line breaks

- declare-function notmuch-unthreaded lacked file name
- declare-function notmuch-search had differently named last arg
  - note: check-declare-directory did not complain about that
- declare-function notmuch-search-show-thread without nil
- some functions declared to be in different file than those
  existed ("notmuch" -> "notmuch-lib")

- some related function/declare lines were (/are now) wider than
  80-columns; added line breaks (and proper indentation) there