s3: Change unix_convert to use an smb_filename struct internally

s3: Change unix_convert to use an smb_filename struct internally

This allows SMB_VFS_[L]STAT to be called directly.  Additionally, I
changed NTSTATUS result to be named status for consistency.

I also removed the stat_cache_add() from build_stream_path() because
stat_cache_lookup() is never actually called on a file with a stream.
There is no reason why the stat cache couldn't be consulted for
streams in the future.

Jeremy/Volker, please take a look at this one when you get a chance.