vfs: resolve absolute symlinks inside their particular backup set.

vfs: resolve absolute symlinks inside their particular backup set.

Let's say you back up a file "/etc/motd" that's a symlink to
"/var/run/motd".  The file inside the backup repo is actually
/whatever/latest/etc/motd, so the symlink should *actually* point to
/whatever/latest/var/run/motd.  Let's resolve it that way automatically in
Symlink.dereference().

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>