Compression header enum

Compression header enum

Define an enum instead of using ELFCOMPRESS_ZLIB and ELFCOMPRESS_ZSTD
in bfd and binutils, and move some functions from bfd.c to compress.c.
When looking at the COFF/PE debug compression support, I wondered
about extending it to support zstd.  I likely won't do that, but
the compression header ch_type field isn't just ELF specific if these
headers are to be used in COFF/PE too.

bfd/
	* bfd.c (bfd_update_compression_header),
	(bfd_check_compression_header, bfd_get_compression_header_size),
	(bfd_convert_section_size, bfd_convert_section_contents): Move to..
	* compress.c: ..here.
	(enum compression_type): New.  Use it throughout file.
	* elf.c (_bfd_elf_make_section_from_shdr): Replace uses of
	ELFCOMPRESS_ZLIB and ELFCOMPRESS_ZSTD with ch_compress_zlib and
	ch_compress_zstd.
	* bfd-in2.h: Regenerate.
binutils/
	* readelf.c (process_section_headers, dump_section_as_strings),
	(dump_section_as_bytes, load_specific_debug_section): Replace
	uses of ELFCOMPRESS_ZLIB and ELFCOMPRESS_ZSTD with
	ch_compress_zlib and ch_compress_zstd.