This is the simplest solution, not the best, but there's significant risk in try to remove the "we have a path" assumption in the code pay code.
Includes removing a `tal_steal` which was incorrect: the buffer has the same lifetime as the plugin, so if we steal it then things get messy when we free the struct payment.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: Plugins: `pay` will now pay your own invoices if you try.
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