Some Tramp adaptions, mainly direct async processes

Some Tramp adaptions, mainly direct async processes

* doc/misc/tramp.texi (Firewalls, Remote processes)
(Frequently Asked Questions): Add @vindex.
(Predefined connection information): Precise precondition or direct
async processes.
(Remote shell setup): Ban ssh RemoteCommand option.
(Frequently Asked Questions): Adapt quoting.

* doc/misc/trampver.texi:
* lisp/net/trampver.el: Change version to "2.5.1-pre".

* lisp/net/tramp-adb.el (tramp-methods) <adb>: Add `tramp-direct-async'
parameter.
(tramp-adb-handle-make-process): Adapt docstring.

* lisp/net/tramp-sh.el (tramp-methods) <scp, scpx, rsync, ssh, sshx>:
Add `tramp-direct-async' parameter.
(tramp-sh-handle-insert-directory): Simplify merkers.
(tramp-sh-handle-make-process): Adapt docstring.

* lisp/net/tramp.el (tramp-methods): Adapt docstring.
(tramp-debug-message): Suppress lockfiles.
(tramp-test-message): New defun.
(tramp-direct-async-process-p): Check also for `tramp-direct-async'.
(tramp-handle-make-process): Do not check for `tramp-direct-async-args'.

* test/lisp/net/tramp-tests.el (all): Replace `string-match' by
`string-match-p'.
(dired-copy-dereference): Declare.
(tramp-test-temporary-file-directory):
Remove `tramp-direct-async-args` for mock method.
(tramp-test15-copy-directory, tramp-test40-special-characters)
(tramp-test40-special-characters-with-stat)
(tramp-test40-special-characters-with-perl)
(tramp-test40-special-characters-with-ls, tramp-test41-utf8)
(tramp-test41-utf8-with-stat, tramp-test41-utf8-with-perl)
(tramp-test41-utf8-with-ls): Skip for tramp-rclone.el.
(tramp--test--deftest-direct-async-process): Do not skip for mock
method.
(tramp-test32-shell-command): Adapt test for direct async processes.
(tramp-test36-vc-registered, tramp--test-hpux-p, tramp--test-ksh-p):
Use `tramp-test-vec'.