Handle process property `remote-command' in Tramp

Handle process property `remote-command' in Tramp

* doc/misc/tramp.texi (Remote processes): New subsection "Process
properties of asynchronous remote processes".

* lisp/net/tramp.el (tramp-handle-make-process):
* lisp/net/tramp-adb.el (tramp-adb-handle-make-process):
* lisp/net/tramp-smb.el (tramp-smb-handle-start-file-process):
* lisp/net/tramp-sh.el (tramp-sh-handle-make-process)
Set `remote-command' process property.
(tramp-scp-direct-remote-copying): Rename connection property.

* test/lisp/net/tramp-tests.el (tramp-test29-start-file-process)
(tramp-test30-make-process, tramp-test31-interrupt-process)
(tramp--test-async-shell-command): Check process property
`remote-command'.