Merge configure --use-xattr and --use-xattr-ref functionality

[bonefish]

When xattr support is available, the attribute emulation should behave like the current --use-xattr, i.e. store the attributes in untyped attributes. Only when for a file the attribute size limit is hit, the emulation should -- for that file only -- fall back to the --use-xattr-ref behavior, i.e. tag the file with a unique ID and store the attributes in separate files. A similar fallback mechanism could be used for symlinks, for which non-super-users cannot write attributes on Linux.

The proposed algorithm should help to reduce the number of attribute files needed (and thus potentially leaked) significantly, since there shouldn't be that many files for which we actually encounter the common attribute size limit.

[waddlesplash]

Unfortunately on most Linux filesystems the limit seems to be around 4KB or so per file (Linux has a hard limit of 64KB per attribute atm) and we seem to use an average of 5-6KB per file. Following my revamp to enable use-xattr or use-xattr-ref, I wonder if this can just be closed?

[nielx]

Assign tickets with status=closed and resolution=fixed within the R1/beta2 development window to the R1/beta2 Milestone