Improve Tramp cache for asynchronous processes

Improve Tramp cache for asynchronous processes

* lisp/net/tramp-adb.el (tramp-adb-handle-exec-path)
(tramp-adb-get-device):
* lisp/net/tramp-gvfs.el (tramp-gvfs-handler-askquestion):
* lisp/net/tramp-sh.el (tramp-remote-selinux-p, tramp-remote-acl-p)
(tramp-open-connection-setup-interactive-shell)
(tramp-maybe-open-connection, tramp-get-remote-path)
(tramp-get-inline-compress, tramp-get-inline-coding):
* lisp/net/tramp-smb.el (tramp-smb-get-cifs-capabilities)
(tramp-smb-get-stat-capability):
* lisp/net/tramp-sudoedit.el (tramp-sudoedit-remote-acl-p)
(tramp-sudoedit-remote-selinux-p): Cache property in main process.

* lisp/net/tramp-cache.el (tramp-cache-undefined): New defconst.
(tramp-get-hash-table, tramp-connection-property-p): Use it.
(tramp-set-connection-property, tramp-flush-connection-property)
(tramp-flush-connection-properties): Add sanity checks.
(tramp-get-file-property, tramp-set-file-property)
(tramp-get-connection-property, tramp-set-connection-property)
(tramp-dump-connection-properties): Adapt docstring.

* lisp/net/tramp-cmds.el (tramp-cleanup-connection): Delete all
processes.

* lisp/net/tramp-gvfs.el (tramp-gvfs-unmount):
Use `tramp-cleanup-connection'.

* lisp/net/tramp-sh.el (tramp-sh-handle-vc-registered):
Use `bound-and-true-p'.

* lisp/net/tramp.el (tramp-get-process): New defun.