Some minor Tramp corrections

Some minor Tramp corrections

* lisp/net/tramp.el (tramp-handle-directory-file-name):
Handle several trailing slashes correctly.
(tramp-handle-file-selinux-context): New defun.

* 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-smb.el (tramp-smb-file-name-handler-alist):
Use `tramp-handle-file-selinux-context'.

* test/lisp/net/tramp-tests.el (tramp-test06-directory-file-name):
Extend test.
(tramp-test17-insert-directory): Make check more robust.
(tramp-test42-auto-load): Combine several let forms.
(tramp-test42-delay-load, tramp-test42-recursive-load)
(tramp-test42-remote-load-path, tramp-test43-unload): Rename.