support cramfs mounts

support cramfs mounts

Somewhat complicated: we have to build the regular directory and file
hierarchy using ext2, then generate a cramfs image and loopback mount it,
and all inside the tests themselves.  So support for cramfs (and similar
special file systems) has to be built into the scaffold lower-level
routines.

Also: in the havechattr test, if "touch" on the branch failed, assume that
there's no chatter support (or that it can't be used, e.g., in a readonly
file system).

Finally, don't show stderr messages in havechattr.