check: make sure all files outside contrib/ include "config.h" first.

And turn "" includes into full-path (which makes it easier to put
config.h first, and finds some cases check-includes.sh missed
previously).

config.h sets _GNU_SOURCE which really needs to be done before any
'#includes': we mainly got away with it with glibc, but other platforms
like Alpine may have stricter requirements.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

bitcoin/script.c

@@ -1,9 +1,10 @@
-#include "address.h"
-#include "locktime.h"
-#include "preimage.h"
-#include "pubkey.h"
-#include "script.h"
+#include "config.h"
 #include <assert.h>
+#include <bitcoin/address.h>
+#include <bitcoin/locktime.h>
+#include <bitcoin/preimage.h>
+#include <bitcoin/pubkey.h>
+#include <bitcoin/script.h>
 #include <ccan/endian/endian.h>
 #include <ccan/mem/mem.h>
 #include <common/utils.h>