Add configurable checks for ASCII and for non-letters

Add configurable checks for ASCII and for non-letters

New boolean settings require_ascii_printable and require_non_letter
are supported in the krb5-strength setting of [appdefaults] in
krb5.conf.  The former rejects passwords containing characters other
than printable ASCII characters (including space), and the latter
requires that passwords contain at least one character that is not a
letter (upper or lower case) or a space.