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-server.el

@@ -45,7 +45,7 @@ session."
 
 Changes to this value only take effect after (re)starting the mu
 session."
-  :type 'file
+  :type '(file :must-match t)
   :group 'mu4e
   :safe 'stringp)