ext2: create a struct for the OCaml 't' type

Use an helper struct for holding the ext2_filsys variable, so that can
be used to add more data.
ext2: store the debug level
ext2: print every file being copied

Print every file copied by supermin, so it is easier to spot whether
some file causes troubles to supermin (e.g. what happens in
RHBZ#1099172).

Since the resulting output is huge, make it valid for debug level 3.