Basic support for BSD and Darwin style chflags (stat flags). Applies
these flags at the end of file restore.
Supports linux style ioctl_iflags(2) in a 2 step process. Flags that
need to be applied prior to writes such as compress and especially no-COW
are applied immediately upon file open.
The flags format is backwards compatible. An attribute starting with a
null byte is used to store flags in the entry attributes table. With
an old version of duplicacy the restore of this attribute should silently
fail (effectively be ignored).
Fixes xattr restore to use O_NOFOLLOW so attributes are applied to symlink.
TODO: Tests, possible option to switch off mutable/append prior to
restore of existing file similar to rsync. Does not apply attributes
or flags to the top most directory.
The exclude by attribute function is broken on non-Darwin POSIX: linux and freebsd.
This is because those xattrs must be prefixed by a legal namespace. The old xattr
library implicitly appended the user namespace to the xattr, but the current
official go pkg does not (which is just as well).
Also fix the test to remove the discordant old xattr dependency and provide
test cases for both darwin and non-darwin POSIX.
