librpc:ndr: Convert NDR flags types to enumerations

librpc:ndr: Convert NDR flags types to enumerations

Using an enumeration rather than ‘uint32_t’ or ‘uint64_t’ means that
debuggers such as gdb can now associate the flag constants with their
respective types. This means that if you have an instance of these flags
types, the debugger will show you the individual flags that make up the
value rather than an inscrutable integer value.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>