Compression tidy and fixes

Tidies:
- Move stuff from bfd-in.h and libbfd.c to compress.c
- Delete COMPRESS_DEBUG from enum compressed_debug_section_type
- Move compress_debug field out of link_info to ld_config.
Fixes:
- Correct test in bfd_convert_section_setup to use obfd flags,
  not ibfd.
- Apply bfd_applicable_file_flags to compression bfd flags added
  by gas and ld to the output bfd.

bfd/
	* bfd-in.h (enum compressed_debug_section_type),
	(struct compressed_type_tuple),
	(bfd_get_compression_algorithm),
	(bfd_get_compression_algorithm_name),
	* libbfd.c (compressed_debug_section_names),
	(bfd_get_compression_algorithm),
	(bfd_get_compression_algorithm_name): Move..
	* compress.c: ..to here, deleting COMPRESS_DEBUG from
	enum compressed_debug_section_type.
	(bfd_convert_section_setup): Test obfd flags not ibfd for
	compression flags.
	* elf.c (elf_fake_sections): Replace link_info->compress_debug
	test with abfd->flags test.
	* bfd-in2.h: Regenerate.
binutils/
	* objcopy.c (copy_file): Tidy setting of bfd compress flags.
	Expand comment.
gas/
	* write.c (compress_debug): Test bfd compress flags rather than
	flag_compress_debug.
	(write_object_file): Apply bfd_applicable_file_flags to compress
	debug flags added to output bfd.
include/
	* bfdlink.h (struct bfd_link_info): Delete compress_debug.
ld/
	* ld.h (ld_config_type): Add compress_debug.
	* emultempl/elf.em: Replace references to link_info.compress_debug
	with config.compress_debug.
	* lexsup.c (elf_static_list_options): Likewise.
	* ldmain.c (main): Likewise.  Apply bfd_applicable_file_flags
	to compress debug flags added to output bfd.