Add `make-nearby-temp-file' and `temporary-file-directory'

Add `make-nearby-temp-file' and `temporary-file-directory'

* doc/lispref/files.texi (Unique File Names):
Introduce `make-nearby-temp-file' and `temporary-file-directory'.
(Magic File Names): Mention `make-nearby-temp-file' and
`temporary-file-directory'.

* etc/NEWS (provided): Mention `make-nearby-temp-file' and
`temporary-file-directory'.

* lisp/files.el (mounted-file-systems): New defcustom.
(temporary-file-directory, make-nearby-temp-file): New defuns.
(normal-backup-enable-predicate): Fix docstring.

* lisp/net/tramp-adb.el (tramp-adb-file-name-handler-alist):
* lisp/net/tramp-gvfs.el (tramp-gvfs-file-name-handler-alist):
* lisp/net/tramp-sh.el (tramp-sh-file-name-handler-alist):
* lisp/net/tramp-smb.el (tramp-smb-file-name-handler-alist):
<make-nearby-temp-file, temporary-file-directory>: Add handler.

* lisp/net/tramp.el (tramp-file-name-for-operation):
Add `make-nearby-temp-file' and `temporary-file-directory'.
(tramp-get-remote-tmpdir): Remove compatibility code.
(tramp-handle-temporary-file-directory)
(tramp-handle-make-nearby-temp-file): New defuns.

* lisp/org/ob-core.el (org-babel-local-file-name):
* lisp/progmodes/gud.el (gud-common-init):
* lisp/vc/vc-hooks.el (vc-user-login-name): Use `file-remote-p'.

* lisp/vc/vc-git.el (vc-git-checkin): Handle remote log message.

* test/lisp/net/tramp-tests.el (tramp-test06-directory-file-name):
Check `tramp--test-enabled'.
(tramp-test18-file-attributes): Add tests for
`file-ownership-preserved-p'.
(tramp-test27-start-file-process, tramp-test28-shell-command):
Reduce timeouts in `accept-process-output'.
(tramp-test--shell-command-to-string-asynchronously): Add timeout.
(tramp-test29-environment-variables): Remove additional sleep calls.
(tramp-test32-make-nearby-temp-file): New test.
(tramp--test-special-characters, tramp--test-utf8): Adapt docstring.
(tramp-test33-special-characters)
(tramp-test33-special-characters-with-stat)
(tramp-test33-special-characters-with-perl)
(tramp-test33-special-characters-with-ls, tramp-test34-utf8)
(tramp-test34-utf8-with-stat, tramp-test34-utf8-with-perl)
(tramp-test34-utf8-with-ls)
(tramp-test35-asynchronous-requests)
(tramp-test36-recursive-load, tramp-test37-unload): Rename.
(tramp--test-ftp-p): Simplify check.
(tramp--test-sh-p): New defun.
(tramp-test20-file-modes, tramp-test22-file-times)
(tramp-test26-process-file, tramp-test27-start-file-process)
(tramp-test28-shell-command)
(tramp-test29-environment-variables)
(tramp-test30-vc-registered)
(tramp-test33-special-characters-with-stat)
(tramp-test33-special-characters-with-perl)
(tramp-test33-special-characters-with-ls)
(tramp-test34-utf8-with-stat, tramp-test34-utf8-with-perl)
(tramp-test34-utf8-with-ls)
(tramp-test35-asynchronous-requests): Use it.