containerfs: rewrite it to use *at functions

do not use absolute paths when accessing files.  Use relative paths to
the layers root.  As part of the refactoring, also drop any cache of
the lower layers, working more similarly as overlay in the kernel.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

README.md

@@ -6,14 +6,7 @@ An implementation of overlay+shiftfs in FUSE for rootless containers.
 Known issues:
 =======================================================
 
-A known issue is that containerfs reports a different inode even
-if the inode on disk is the same, as with hard links.
-
-The current implementation keeps in memory the entire tree of the
-lower directories.  A side effect is that modifications to the
-lower layers is never propagated to the merged directory.  Overlayfs
-doesn't allow changes to the layers as well, altough since there is
-not such caching done, changes are usually propagated.
+Hard links in some cases show different inode values.
 
 Read-only mode is not supported, so it is always required to specify
 an upperdir and a workingdir.