Some Tramp changes, mainly in tramp-tests.el

Some Tramp changes, mainly in tramp-tests.el

* doc/misc/tramp.texi (External packages): Don't use Tramp internals.

* lisp/net/tramp-gvfs.el (tramp-gvfs-handler-mounted-unmounted):
Protect `tramp-make-tramp-file-name' call.

* lisp/net/tramp.el (tramp-make-tramp-file-name): Set advertised
calling conventions.

* test/lisp/net/tramp-tests.el (tramp-test18-file-attributes):
Adapt test.
(tramp--test-supports-processes-p): New defun.
(tramp-test28-process-file, tramp-test29-start-file-process)
(tramp-test30-make-process, tramp-test32-shell-command)
(tramp-test32-shell-command-dont-erase-buffer)
(tramp-test34-explicit-shell-file-name, tramp-test35-exec-path)
(tramp-test44-asynchronous-requests): Use it.