smb: disable mode bits getting forwarded to KIO

Summary:
mode bits in libsmb_stat are fairly cheaply constructed. file and directory
qualification is alright, beyond that it is a bit of a shambles.
specifically +W is set iff the DOS attribute READONLY isn't set, but
that attribute doesn't mean what we think it means, at least not on
NT+ systems. see exhaustive comment.

long story short: we cannot represent mode bits accurately because NT's
access controls simply do not map to posix mode bits. therefore I'm
removing the mode setting for the udsentry, making all entries effectively
writable.

whether an entry truly is writable is hard to say anyway. specifically
because SMB+NTFS have independent ACLs. so, the SMB ACL may allow a
given user or group to do something, that doesn't mean they'll also
have permissions on a file system level.

BUG: 414482
FIXED-IN: 19.12.3

Test Plan:
previously not writable shares as described in the bug report are now writable.
permission dialog looks a bit meh now, it wasn't really designed around not knowing
what the access situation is. gnome simply shows a "dunno what permissions we have" label
instead of everything, that seems like a reasonable approach

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: dfaure, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D27372