Now we wire in the code which gathers configvars and parses from there;
lightningd keeps the array of configuration variables for future use.
Note that lightning-cli also needs to read the config, but it has its
own options (including short ones!) and doesn't want to use this
configvar mechanism, so we have a different API for that now.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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