* libbfd.c: Add signed versions of bfd_{h_,}{get,put}_signed_<size>.

	* libbfd.c: Add signed versions of bfd_{h_,}{get,put}_signed_<size>.
	libbfd.c, libbfd-in.h: Add _do*signed*.
	targets.c, all targets: Add bfd*signed*.
	bfd-in.h: Add bfd_signed_vma.  Add comments.