support ZSTD compression algorithm

support ZSTD compression algorithm

config/ChangeLog:

	* libelf.pc.in: Add LIBLZSTD to Requires.private.

ChangeLog:

	* configure.ac: Detect ZSTD streaming API.

libelf/ChangeLog:

	* Makefile.am: Use zstd_LIBS.
	* elf_compress.c:
	(__libelf_compress): Split into ...
	(__libelf_compress_zlib): ... this.
	(do_zstd_cleanup): New.
	(zstd_cleanup): New.
	(__libelf_compress_zstd): New.
	(__libelf_decompress): Switch in between zlib and zstd.
	(__libelf_decompress_zlib): Renamed from __libelf_decompress.
	(__libelf_decompress_zstd): New.
	(__libelf_decompress_elf): Dispatch in between compression
	algorithms.
	(elf_compress): Likewise.
	* elf_compress_gnu.c (elf_compress_gnu): Call with
	ELFCOMPRESS_ZLIB.
	* libelfP.h (__libelf_compress): Add new argument.
	(__libelf_decompress): Add chtype argument.

src/ChangeLog:

	* elfcompress.c (enum ch_type): Add ZSTD.
	(parse_opt): Parse "zstd".
	(get_section_chtype): New.
	(process_file): Support zstd compression.
	(main): Add zstd to help.
	* readelf.c (elf_ch_type_name): Rewrite with switch.

tests/ChangeLog:

	* Makefile.am: Add ELFUTILS_ZSTD if zstd is enabled.
	* run-compress-test.sh: Test zstd compression algorithm
	for debug sections.