Fix bug #15277 - MacOSX clients fail against an SMB2 DFS share.

Windows, Linux and libsmbclient/smbclient all send SMB2 DFS pathnames as SERVER\SHARE\path\to\file.

Turns out that MacOSX sends \SERVER\SHARE\path\to\file (note the leading '\').

First add a regression test to reproduce the problem + knownfail, then fix smbd (4.17 and above) to cope with this (remove knownfail). Passes against Windows as it turns out that Windows strips all leading '\' characters from an incoming SMB2 DFS path before processing further.

In my defense I didn't catch this when I added all the new DFS tests as I'd never seen a client send a leading '\' character over SMB2 for a DFS share (and it's explicitly specified as illegal for a non-DFS share). Now I have so we test for this (and yes, I test with multiple leading '\' characters too :-).

This will make it easier in future to fix our SMB2 DFS path processing to strip out all the DFS cruft before passing to our normal filename processing (i.e. eventually I will be able to remove dfs_filename_convert() from filename.c). But that is a more complex fix for another day.

Passes ci.