Fix defcustom type definitions

So that the type definitions reflects the docstrings more closely.  In additon,
fixed warnings from flymake about docstring.

* mu4e/mu4e-draft.el
mu4e-compose-keep-self-cc: should be a comma

* mu4e/mu4e-folders.el
mu4e-maildir-shortcuts: recognize new (plist) format

* mu4e/mu4e-helpers.el
mu4e-debug: removed unnecessary double space in docstring

* mu4e/mu4e-query-items.el
mu4e-query-rewrite-function: prepend "\\=" before column 0 "(" in docstring

* mu4e/mu4e-search.el
mu4e-search-sort-field mu4e-search-sort-direction: used radio in types

* mu4e/mu4e-server.el
mu4e-mu-binary: "mu" executable must exist

* mu4e/mu4e-view.el
mu4e-view-fields: fixed type
mu4e-view-actions: for function refs, prefer "#'" rather than "'"

mu4e/mu4e-view.el

@@ -69,11 +69,11 @@ Note, you can use this to add fields that are not otherwise
 shown; you can further tweak the other fields using e.g.,
 `gnus-article-hide-boring-headers', `gnus-article-hide-headers'
 etc., see the gnus documentation for details."
-  :type (list 'symbol)
+  :type '(repeat symbol)
   :group 'mu4e-view)
 
 (defcustom mu4e-view-actions
-  (seq-filter 'identity
+  (seq-filter #'identity
               `( ("capture message"  . mu4e-action-capture-message)
                  ("view in browser"  . mu4e-action-view-in-browser)
                  ,(when (fboundp 'xwidget-webkit-browse-url)