XATTRs are not connected to Windows EA about zfsin HOT 3 CLOSED

ok, So query_information(FileEaInformation) and set_file_information(FileEaInformation) seem to be a legacy API for small xattrs. Combined maximum length of 0xffff.

The functions come in as "binary blob" - similar to readdir, each entry has a next offset, name length, value length followed by name and value, there is some 4 byte padding between structs.

Our code should iterate through the struct, and call the ZFS APIs zpl_xattr_set_sa() zfs_get_xattrdir() zfs_obtain_xattr and zfs_read/zfs_write to deal with the value.

readdir should also be able to return EAs, which could be a pain.

If a valuelen is 0, it should remove xattr (nvlist_remove / zfs_remove).

Full XATTRs appear to be ADS (alternate data stream) - also discussed in #34

ADS are encoded in the name, as in filename.txt:streamname:type - where type is NTFS specific (like $OBJECT_ID, $EA, $BITMAP, ...) but we should at least handle $DATA. The default, normal datastream has no name, so is filename.txt::$DATA.

Apart from the filename parsing, converting streams into ZFS XATTRs should be easy, as they are almost the same.

https://docs.microsoft.com/en-us/windows/desktop/fileio/file-streams

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#100

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Merged with master.

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