Rework direct async processes in Tramp

Rework direct async processes in Tramp

* doc/misc/tramp.texi (Remote processes): Precise restrictions for direct
async processes.

* lisp/net/tramp.el (tramp-methods): Adapt docstring.
(tramp-direct-async-process-p): Make it more precise.
(tramp-handle-make-process): Rewrite, based on `make-process'.

* test/lisp/net/tramp-tests.el (tramp-test-temporary-file-directory):
Add `tramp-direct-async-args` for mock method.
(tramp-test29-start-file-process, tramp-test30-make-process):
Use weaker regexp checking "foo".
(tramp-test30-make-process): Do not check stderr for direct async processes.
(tramp--test--deftest-direct-async-process): New defmacro.
(tramp-test29-start-file-process-direct-async)
(tramp-test30-make-process-direct-async): New tests.