Tramp's direct asynchronous processes use 'tramp-remote-path'

Tramp's direct asynchronous processes use 'tramp-remote-path'

* doc/misc/tramp.texi (Remote processes): Remove item about
tramp-remote-path.

* etc/NEWS: Direct asynchronous processes use 'tramp-remote-path'.

* lisp/net/tramp-sh.el (tramp-get-remote-pipe-buf): New defun.
(tramp-set-remote-path): Use it.
(tramp-get-remote-path): Add ;;;###tramp-autoload cookie.

* lisp/net/tramp.el (tramp-handle-make-process):
Use `tramp-remote-path' for setting PATH environment.

* test/lisp/net/tramp-tests.el (tramp-test35-exec-path-direct-async)
(tramp-test35-remote-path-direct-async): New tests.