libext2fs: add rbtree library

libext2fs: add rbtree library

This commit adds rbtree library into e2fsprogs so it can be used for
various internal data structures. The rbtree implementation is ripped of
kernel rbtree implementation with small changes needed for it to work
outside kernel.

[ I prefixed the exported symbols and interface with ext2fs_ to keep
  avoid pulluting the namespace exported by the libext2fs shared
  library.  -- tytso ]

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>