Add 'remote-file-error' for Tramp

Add 'remote-file-error' for Tramp

* doc/lispref/errors.texi (Standard Errors): Add 'remote-file-error'.

* etc/NEWS: Mention 'remote-file-error'.

* lisp/net/ange-ftp.el (ftp-error): Add error condition
`remote-file-error'.

* lisp/net/tramp-cmds.el (tramp-cleanup-all-connections): Do not set
`tramp-locked'.

* lisp/net/tramp-compat.el (remote-file-error): Define if it
doesn't exist.

* lisp/net/tramp-sh.el (tramp-timeout-session): Check for "locked"
property.
(tramp-maybe-open-connection): Simplify.

* lisp/net/tramp.el (tramp-locked, tramp-locker): Remove them.
(tramp-file-name-handler): Do not set them.
(with-tramp-locked-connection): New defmacro.
(tramp-accept-process-output, tramp-send-string): Use it.

* src/fileio.c (Qremote_file_error): New error symbol.

* test/lisp/net/tramp-tests.el (tramp-test43-asynchronous-requests):
Adapt test.  Remove :unstable tag.