Add connection-local variable `tramp-direct-async-process'

Add connection-local variable `tramp-direct-async-process'

* doc/misc/tramp.texi (Predefined connection information):
Remove "direct-async-process".
(Remote processes): Explain connection-local variable
tramp-direct-async-process.

* etc/NEWS: Add connection-local variable 'tramp-direct-async-process'.

* lisp/net/tramp-compat.el (tramp-compat-connection-local-p): Sync with
Emacs source.
(tramp-compat-connection-local-value): New defalias.

* lisp/net/tramp-message.el (tramp-warning): New defsubst.

* lisp/net/tramp.el (tramp-direct-async-process): New defvar.
(tramp-direct-async-process-p): Use connection-local variable for
check.  (Bug#70959)

* test/lisp/net/tramp-tests.el
(tramp--test-deftest-direct-async-process):
Use connection-local-variable `tramp-direct-async-process'.