[RTL] Introduce RtlpImageNtHeader,

which implements the required functionality.
ntdll and ntoskrnl now have a wrapper for this, with SEH.
This protects the function against malformed / bad images,
whilst still being able to use the code in freeldr et al.
Idea from Thomas.
CORE-14857

boot/environ/lib/rtl/libsupp.c

@@ -0,0 +1,39 @@
+/*
+ * COPYRIGHT:       See COPYING.ARM in the top level directory
+ * PROJECT:         ReactOS UEFI Boot Library
+ * FILE:            boot/environ/lib/rtl/libsupp.c
+ * PURPOSE:         RTL Support Routines
+ * PROGRAMMER:      Mark Jansen (mark.jansen@reactos.org)
+ */
+
+/* INCLUDES ******************************************************************/
+
+#include "bl.h"
+
+/* FUNCTIONS *****************************************************************/
+
+/* Ldr access to IMAGE_NT_HEADERS without SEH */
+
+/* Rtl SEH-Free version of this */
+NTSTATUS
+NTAPI
+RtlpImageNtHeaderEx(
+    _In_ ULONG Flags,
+    _In_ PVOID Base,
+    _In_ ULONG64 Size,
+    _Out_ PIMAGE_NT_HEADERS *OutHeaders);
+
+
+/*
+ * @implemented
+ */
+NTSTATUS
+NTAPI
+RtlImageNtHeaderEx(
+    _In_ ULONG Flags,
+    _In_ PVOID Base,
+    _In_ ULONG64 Size,
+    _Out_ PIMAGE_NT_HEADERS *OutHeaders)
+{
+    return RtlpImageNtHeaderEx(Flags, Base, Size, OutHeaders);
+}