Improve handling of file modes in Tramp

Improve handling of file modes in Tramp

* lisp/net/tramp-gvfs.el (tramp-gvfs-handle-get-remote-uid)
(tramp-gvfs-handle-get-remote-gid): Do not assume that the default
location is owned be the remote uid/gid.
(tramp-gvfs-handle-file-executable-p):
* lisp/net/tramp-sh.el (tramp-sh-handle-file-executable-p):
Check also for setuid/setgid bit.

* lisp/net/tramp.el (tramp-check-cached-permissions): Check also
for ?s access type.  Check whether remote uid/gid are unknown.