[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/freeldr/freeldr/lib/rtl/libsupp.c

@@ -57,3 +57,30 @@ RtlpSafeCopyMemory(
     RtlCopyMemory(Destination, Source, Length);
     return STATUS_SUCCESS;
 }
+
+/* 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);
+}
+