Follow convention in which each NLM header has an 8 byte stamp

	Follow convention in which each NLM header has an 8 byte stamp
	followed by a four byte length.
	* libnlm.h (struct nlm_obj_tdata): Rename nlm_cygnus_section_hdr
	to nlm_cygnus_ext_header, and change type to
	Nlm_Internal_Cygnus_Ext_Header.
	(nlm_cygnus_ext_header): Rename from nlm_cygnus_section_header.
	* nlmcode.h (nlm_swap_auxiliary_headers_in): Use CyGnUsEx instead
	of CyGnUsSeCs.  Rename from cygnus_section to cygnus_ext.  Require
	length word to be 8.
	(nlm_swap_auxiliary_headers_out): Rename from cygnus_section to
	cygnus_ext.  Set length word to 8.
	(nlm_compute_section_file_positions): Rename from cygnus_section
	to cygnus_ext.