libbacktrace: simplify DWARF section handling

This is in preparation for adding DWARF 5 support.

	* internal.h (enum dwarf_section): Define.
	(struct dwarf_sections): Define.
	(backtrace_dwarf_add): Update declaration to replace specific
	section parameters with dwarf_sections parameter.
	* dwarf.c (struct dwarf_data): Replace specific section fields
	with dwarf_sections field.
	(read_attribute): Use dwarf_sections with altlink.
	(build_address_map): Replace specific section parameters with
	dwarf_sections parameter.  Change all callers.
	(read_line_info): Use dwarf_sections with ddata.
	(read_referenced_name): Likewise.
	(add_function_ranges): Likewise.
	(read_function_entry): Likewise.
	(read_function_info): Likewise.
	(build_dwarf_data): Replace specific section parameters with
	dwarf_sections parameter.  Change all callers.
	(backtrace_dwarf_add): Likewise.
	* elf.c (enum debug_section): Remove.
	(dwarf_section_names): Remove .zdebug names.
	(elf_add): Track zsections separately.  Build dwarf_sections.
	* pecoff.c (enum debug_section): Remove.
	(struct debug_section_info): Remove data field.
	(coff_add): Build dwarf_sections.
	* xcoff.c (enum dwarf_section): Remove.  Replace DWSECT_xxx
	references with DEBUG_xxx references.
	(xcoff_add): Build dwarf_sections.

From-SVN: r278984