libext2fs: implement NLS framework support

libext2fs: implement NLS framework support

Basic NLS support is required in e2fsprogs because of fsck, which
needsto calculate dx hashes for encoding aware filesystems.  this patch
implements this infrastructure as well as ascii support.

We don't need to do all the dance of versioning as we do in the kernel,
because we know before-hand which encodings and versions we
support (those we know how to store in the sb), so it is simpler just to
create static tables.

This version only has support for ASCII; we'll add support for Unicode
later.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>