Fix rc4 string-to-key on unterminated inputs

The internal UTF-8 to UCS-2 conversion functions did not properly
respect their length arguments, instead assuming that the input string
is terminated with a zero bytes.  As a result,
krb5int_arcfour_string_to_key could fail on unterminated inputs.  Fix
the underlying support functions to read their inputs only up to the
specified length.

ticket: 7643 (new)
Make empty passwords work via init_creds APIs

In the gak_data value used by krb5_get_as_key_password, separate the
already-known password from the storage we might have allocated to put
it in, so that we no longer use an empty data buffer to determine
whether we know the password.  This allows empty passwords to work via
the API.

Remove the kadm5 test which explicitly tests with an empty password.

Based on a patch from Stef Walter.

ticket: 7642