[RFC] Implement file attributes with xattr on mac and linux (fixes bug 15679)

[Dan Kegel]

Builds and runs on mac and linux,
passes tests on linux and vista.
Fixes http://bugs.winehq.org/show_bug.cgi?id=15679

It's worth discussing a bit before posting to wine-patches.
This patch uses filesystem extended attributes to
store the win32 attribute bits.  These are available
fairly widely finally - I think current Linux, Mac, and Solaris
all support some form of extended attributes.

Because the extended attribute API varies somewhat wildly
between operating systems, I went whole-hog and used
autoconf to detect the differences between the api on
linux and macosx.  Somebody else should be able to
add solaris support without much difficulty if this is approved.

The convention chosen is that the win32 file attributes
are stored as a four digit hex number in the extended
attribute named user.wine.attr.  (The 'user' prefix is suggested
by http://www.freedesktop.org/wiki/CommonExtendedAttributes .)

For backwards compatibility,
if a native file lacks extended attributes; it is reported as having
ARCHIVE attribute.
Likewise, if a file just has the ARCHIVE attribute set, there is no
native extended attribute.

Here's an example:
  touch foo.dat
  wine cmd /c attrib -a foo.dat
  xattr -l foo.dat
This outputs
  user.wine.attr: 0000

i.e. because the file has anything but the default attribute, we store
the entire win32 attribute bitmask in the extended attribute.

Another example:
  touch bar.dat
  wine cmd /c attrib +s bar.dat
  xattr -l bar.dat
This outputs
  user.wine.attr: 0024
or ARCHIVE | SYSTEM.

This convention should be thought over carefully before we go with it.
I also considered using individual xattrs for each attribute bit,
but discarded that idea as adding overhead and complexity
without any obvious payoff.

Finally, the implementation of attrib in cmd is lame -- it only handles
two arguments.  It's enough for the moment, but the real attrib
can handle multiple attribute flags, and we should support that sometime.

So, what do folks think?
- Dan
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