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-query-items.el

@@ -46,7 +46,7 @@ searching, and allows users to modify the query.
 For instance, we could change any instance of \"workmail\" into
 \"maildir:/long-path-to-work-related-emails\", by setting the function
 
-(setq mu4e-query-rewrite-function
+\\=(setq mu4e-query-rewrite-function
   (lambda(expr)
      (replace-regexp-in-string \"workmail\"
                    \"maildir:/long-path-to-work-related-emails\" expr)))