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-search.el

@@ -105,13 +105,20 @@ one of: `:date', `:subject', `:size', `:prio', `:from', `:to.',
 Note that when threading is enabled (through
 `mu4e-search-threads'), the headers are exclusively sorted
 chronologically (`:date') by the newest message in the thread."
-  :type 'symbol
+  :type '(radio (const :date)
+                (const :subject)
+                (const :size)
+                (const :prio)
+                (const :from)
+                (const :to)
+                (const :list))
   :group 'mu4e-search)
 
 (defcustom mu4e-search-sort-direction 'descending
   "Direction to sort by; a symbol either `descending' (sorting
   Z->A) or `ascending' (sorting A->Z)."
-  :type 'symbol
+  :type '(radio (const ascending)
+                (const descending))
   :group 'mu4e-search)
 
 ;; mu4e-query-rewrite-function lives in mu4e-query-items.el