Go back to grave quoting in Tramp

Go back to grave quoting in Tramp

* lisp/net/tramp-adb.el:
* lisp/net/tramp-cache.el:
* lisp/net/tramp-compat.el:
* lisp/net/tramp-gvfs.el:
* lisp/net/tramp-gw.el:
* lisp/net/tramp-sh.el:
* lisp/net/tramp-smb.el:
* lisp/net/tramp.el:
Stick with grave quoting in diagnostics strings.  This is more
portable to older Emacs, desirable for Tramp.
* lisp/net/tramp-cache.el: Use ‘format-message’, not ‘format’,
for diagnostic that needs requoting.
* lisp/net/tramp-compat.el (format-message):
Fall back on simple ‘format’, since that’s good enough now.