Properly reflect RENAME ops in FUSE internal state

In response to a RENAME, we actually need to rename and move the virtual
node.  To support this, filenames are now allocated separately, as reallocing
the whole node to accommodate a longer filename would break the direct
mapping of fhs and inodes to fuse pointers.

Change-Id: I71e5a965f875dedc5f58f9d182156734b29ca179