Optimize host name completion in Tramp

Optimize host name completion in Tramp

* lisp/net/tramp-adb.el (tramp-adb-maybe-open-connection):
* lisp/net/tramp-gvfs.el (tramp-gvfs-maybe-open-connection):
* lisp/net/tramp-rclone.el (tramp-rclone-maybe-open-connection):
* lisp/net/tramp-sh.el (tramp-maybe-open-connection):
* lisp/net/tramp-smb.el (tramp-smb-maybe-open-connection):
* lisp/net/tramp-sudoedit.el (tramp-sudoedit-maybe-open-connection):
Throw `non-essential' at the beginning of the function.

* lisp/net/tramp.el (tramp-handle-file-exists-p):
* lisp/net/tramp-sh.el (tramp-sh-handle-file-exists-p):
* lisp/net/tramp-sudoedit.el (tramp-sudoedit-handle-file-exists-p):
Run only when host is connectable.  This is due to host name
completion, which shall be optimized.

* lisp/net/tramp-smb.el (tramp-smb-do-file-attributes-with-stat)
(tramp-smb-get-file-entries): Access connection buffer only after
sending the command.

* lisp/net/tramp.el (tramp-get-buffer, tramp-get-connection-buffer):
New argument DONT-CREATE.
(tramp-message): Use it.
(tramp-get-mutex): Check, whether host is connectable.
(tramp-file-name-handler): Set thread only when host is connectable.
(tramp-connectable-p): Allow also VEC as argument.
(tramp-completion-handle-file-name-completion): Do not expand directory.