Couple of minor fixes for POSIX pathname processing in the

Couple of minor fixes for POSIX pathname processing in the
new stream code. (1) In smbd/filename, don't split the name at
':' if we know it's a posix path (this should be parameterized....).
(2). When calling posix_mkdir, we get the flag FILE_FLAG_POSIX_SEMANTICS
passed to open_directory(). I know for a posix client lp_posix_pathnames
should be true (which is checked for in is_ntfs_stream_name() but we
have an explicit flag here, so let's use it.
Jeremy.