lightningd: switch parsing to common/configvar

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>

cli/test/run-large-input.c

@@ -3,6 +3,7 @@
 #include <common/amount.h>
 #include <common/bigsize.h>
 #include <common/channel_id.h>
+#include <common/configvar.h>
 #include <common/json_stream.h>
 #include <common/node_id.h>
 #include <common/setup.h>