I added a plugin arg and was surprised that compile didn't break.
This is because typesafe_cb et al are conditional casts: if the type
isn't as expected it has no effect, but we're passing plugin_option() through
varargs, so everything is accepted!
Add a noop inline to check type, and fix up the two cases where we
used `const char *` instead of `char *`.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
aa5c7e763f