Improved sys_fs module

- cellFsStat now accepts mounting points in the path argument.

- cellFsMkdir, cellFsRmdir, cellFsUnlink and cellFsRename are now using
the real path provided by the VFS.

- cellFsFtruncate, cellFsTruncate and cellFsFGetBlockSize are partially
implemented.

Note: I have spotted a bug in cellFsRename and cellFsRmdir (this has
nothing to do with the changes I made), but I don't dare to fix it since
I am not sure if I will break other functions with my solution.