libplugin: make set callback for options take plugin ptr, check correct type.

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>

tests/plugins/test_libplugin.c

@@ -6,7 +6,7 @@
 #include <common/memleak.h>
 #include <plugins/libplugin.h>
 
-static const char *somearg;
+static char *somearg;
 static bool self_disable = false;
 static bool dont_shutdown = false;