Add support for reading msdos MZ executables.

Add support for reading msdos MZ executables.

See email thread starting here:  https://www.sourceware.org/ml/binutils/2018-01/msg00001.html

include	* coff/msdos.h: New header.
	* coff/pe.h: Move common defines to msdos.h.
	* coff/powerpc.h: Likewise.

bfd	* i386msdos.c (msdos_mkobject); New function.
	(msdos_object_p): New function.
	(i386_msdos_vec): Use msdos_object_p as the check_format
	function.
	* peicode.h: Rename external_PEI_DOS_hdr, DOSMAGIC, and
	NT_SIGNATURE to external_DOS_hdr, IMAGE_DOS_SIGNATURE, and
	IMAGE_NT_SIGNATURE.
	* peXXigen.c: Likewise.
	* coff-ia64.c: Likewise.