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>

common/utils.c

@@ -1,9 +1,10 @@
-#include "utils.h"
+#include "config.h"
 #include <bitcoin/chainparams.h>
 #include <ccan/list/list.h>
 #include <ccan/str/hex/hex.h>
 #include <ccan/tal/path/path.h>
 #include <ccan/utf8/utf8.h>
+#include <common/utils.h>
 #include <errno.h>
 #include <locale.h>