Explicitly set TRAMP default method

Using sshx as default on non-Windows systems seems to be a more robust choice
than using ssh, because the latter might be prone to errors caused by my
user-specific shell configuration.  And while we are at it, using plink on
Windows is more reasonable than using ssh, at least in my working environment.

init.el

@@ -559,7 +559,8 @@
               browse-url-generic-program "firefox"))
 
 (use-package tramp
-  :init (setq tramp-save-ad-hoc-proxies t))
+  :init (setq tramp-save-ad-hoc-proxies t
+              tramp-default-method (if on-windows "plink" "sshx")))
 
 (use-package calc
   ;; https://florian.adamsky.it/2016/03/31/emacs-calc-for-programmers-and-cs.html