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.
Main changes:
* Change the listing order of files/directories so that the local and remote
snapshots can be compared on-the-fly.
* Introduce a new struct called EntryList that maintains a list of
files/directories, which are kept in memory when the number is lower, and
serialized into a file when there are too many.
* EntryList can also be turned into an on-disk incomplete snapshot quickly,
to support fast-resume on next run.
* ChunkOperator can now download and upload chunks, thus replacing original
ChunkDownloader and ChunkUploader. The new ChunkDownloader is only used
to prefetch chunks during the restore operation.
Directories containing a file with this name will not be backed up. I find it easier to drop a .nobackup file in directories I don't want backed up instead of maintaining a file of exclusions. This is also useful for scripts that create data in the repository but don't want it to be backed up.
