nls: add a nls_nullsize inline

It's possible for character sets to require a multi-byte null
string terminator. Add a helper function that determines the size
of the null terminator at runtime.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>