Enhancement needed to UNIX extensions to correctly support Linux EA's.

[Jeremy Allison <jra-eUNUBHrolfbYtjvyW6yDsg@public.gmane.org>]

See bug https://bugzilla.samba.org/show_bug.cgi?id=9743

No way to set zero length EA's with CIFS vfs.

I was doing some investigations around bug #9130, where Windows fails to copy a
file when we return a zero-length EA associated with it.

Looking carefully into this this is a deficiency in the CIFS UNIX protocols. On
Linux it's possible to set a zero-length EA on a file/directory as a tag of
some kind, with no associated data.

On Windows this is impossible - trying to set a zero-length EA will just delete
it, plus the EA names are case insensitive (and always returned uppercase).

We need to add a UNIX_GET_EAS call that can return all POSIX EA's without
mangling case, and can also return zero-length EA's, also we need a
UNIX_SET_EAS call that can set a zero length EA, and also has an additional
'flags' field that can delete an EA if set.

Please discuss,

Jeremy.