libroot_build: Improve/fix get_path()

In fs_attr_generic() get_path() is used with a symlink FD, which didn't
work at all. Handle symlink descriptors explicitly now.
Add basic BUuid class

ATM it's only able to generate random UUIDs.
Add more robust generic attribute emulation variant

The new configure option "--use-xattr-ref" enables an xattr assisted
variant of the generic attribute emulation. Instead of using the inode
ID of a node to identify its attribute directory, we use a reasonably
unique random 128 bit number, which we generate and attach as an
attribute to the node. This way, when a node changes its inode ID
(defragmentation?) or the inode ID of a removed node with a left-over
attribute directory is reused, attributes won't get mixed up.

The old method is still used for symlinks (since on Linux only
priviledged users can write attributes on symlinks), but those usually
only have a rather boring BEOS:TYPE attribute, so mix-ups wouldn't be
that problematic anyway.