print "mostly-printable" xattr strings in debugfs

print "mostly-printable" xattr strings in debugfs

Taking a cue from getfattr... if a string is "mostly"
printable characters, go ahead & print as a string,
and escape what's left over.

so we get:

Extended attributes stored in inode body:
  selinux = "system_u:object_r:root_t:s0\000" (28)

instead of:

Extended attributes stored in inode body:
  selinux = "73 79 73 74 65 6d 5f 75 3a 6f 62 6a 65 63 74 5f 72 3a 72 6f 6f 74 5f 74 3a 73 30 00 " (28)

(selinux includes the trailing null in "len" so it
never prints as a string today)

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>