debugfs: build read-only variant of debugfs

debugfs: build read-only variant of debugfs

Create a version of debugfs which only supports read-only examination
of the file system metadata (but not the data blocks).  The idea is
that this version of debugfs might be suitable to be setuid root, and
executable only by members of a particular group, or setgid disk, and
globally executable, depending on the security/privacy policies in
force at a particular site.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>