r9338: fixed the winreg IDL to be correct for the EnumKey and EnumValue

r9338: fixed the winreg IDL to be correct for the EnumKey and EnumValue
calls. The previous IDL was just a workaround for the limitations of
our older rpc infrastructure. Now that Jelmer has added much improved
string support using the charset keyword we can correctly implemenent
the unusual winreg string buffers.

Jelmer, note the little comment I put on winreg_StringBuf() about why
I couldn't use [value()] for the length field.

This also fixes EnumKey() and EnumValue() to use NTTIME fields for the
last_changed_time. I don't know why we were using a pair of uint32's,
as it is just a NTTIME.