Work on Tramp's (symbolic) links

Work on Tramp's (symbolic) links

* doc/misc/tramp.texi (Traces and Profiles): Mention the
backtrace when tramp-verbose is greater than or equal to 10.

* lisp/net/tramp-adb.el (tramp-adb-file-name-handler-alist):
Use `tramp-handle-add-name-to-file'.

* lisp/net/tramp-gvfs.el (tramp-gvfs-file-name-handler-alist): Use
`tramp-handle-add-name-to-file' and `tramp-handle-file-truename'.

* lisp/net/tramp-sh.el (tramp-sh-handle-make-symbolic-link): Improve.

* lisp/net/tramp-smb.el (tramp-smb-errors):
Add "NT_STATUS_CONNECTION_DISCONNECTED" and
"NT_STATUS_OBJECT_PATH_SYNTAX_BAD".
(tramp-smb-file-name-handler-alist): Use `tramp-handle-file-truename'.
(tramp-smb-do-file-attributes-with-stat): Return non-nil only
if one of the attributes is non-nil.
(tramp-smb-handle-file-local-copy): Use `file-truename'.
(tramp-smb-handle-file-truename): Move to tramp.el.
(tramp-smb-handle-insert-directory): Show symlinks.
(tramp-smb-handle-make-symbolic-link): Improve.
(tramp-smb-read-file-entry): Handle extended file modes in Samba.

* lisp/net/tramp.el (tramp-handle-add-name-to-file)
(tramp-handle-file-truename): New defuns.

* test/lisp/net/tramp-tests.el (tramp-test21-file-links): Extend test.
(tramp--test-check-files): Make check for "smb".