additional bounds-checking on section sizes

This adds additional bounds-checking on the section sizes. Also adds
-Wsign-compare to the Makefile and replaces some signed variables with
unsigned counteparts for robustness.

Signed-off-by: Kees Cook <kees@ubuntu.com>
Kees' patch missed the offset adjustment to PEHdr.

In read_header, we adjust context->PEHdr's address by doshdr->e_lfanew.
If we're going to recompute that address, we have to adjust it here
too.

Signed-off-by: Peter Jones <pjones@redhat.com>
Get rid of SectionCache in generate_hash(), it is unused.

Signed-off-by: Peter Jones <pjones@redhat.com>