s3-net_rpc: Use security_token_initialise() to create struct security_token

s3-net_rpc: Use security_token_initialise() to create struct security_token

This ensures that the full structure is initialised now and in the
future.

Because this is now a talloc based structure, we can now use
add_sid_to_array_unique() rather than a reimplementation in this file.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>