Tweak Tramp method definition syntax to allow overriding check for localhost-only methods

lisp/ChangeLog

@@ -1,3 +1,10 @@
+2013-10-29  Daniel Colascione  <dancol@dancol.org>
+
+	* net/tramp.el (tramp-methods): Document new functionality.
+	* net/tramp-sh.el (tramp-compute-multi-hops): Punt to
+	tramp-hostname-checker if method provides one instead of scanning
+	argument list for "%h" to decide hostname acceptability.
+
 2013-10-28  Michael Albinus  <michael.albinus@gmx.de>
 
 	* net/tramp-sh.el (tramp-sh-handle-copy-directory):

lisp/net/tramp-sh.el

@@ -4285,6 +4285,10 @@ Gateway hops are already opened."
 	  (or
 	   ;; There are multi-hops.
 	   (cdr target-alist)
+           ;; This method explicitly has an explicit allowability check.
+           (let ((checker (tramp-get-method-parameter
+                           method 'tramp-hostname-checker)))
+             (when checker (funcall checker v host method) t))
 	   ;; The host name is used for the remote shell command.
 	   (member
 	    '("%h") (tramp-get-method-parameter method 'tramp-login-args))

lisp/net/tramp.el

@@ -265,6 +265,15 @@ pair of the form (KEY VALUE).  The following KEYs are defined:
     In general, the global default value shall be used, but for
     some methods, like \"su\" or \"sudo\", a shorter timeout
     might be desirable.
+  * `tramp-hostname-checker'
+    This is a function that tramp calls while setting
+    up a connection.  It is called with three arguments:
+    the target, the host, and the method description.  If
+    the hostname is unacceptable, this function should signal
+    using `tramp-error'.  If a method does not provide
+    a value here, then Tramp looks at whether the method's
+    login program uses a \"%h\" parameter.  If not, then Tramp
+    requires that the given hostname match `tramp-local-host-regexp'. 
 
 What does all this mean?  Well, you should specify `tramp-login-program'
 for all methods; this program is used to log in to the remote site.  Then,