libdwfl: Add ZSTD support.

libdwfl: Add ZSTD support.

Newer kernels might be compressed using ZSTD add support to libdwfl open
so we can can automatically read ZSTD compressed files and kernel images.

The support is very similar to the bzip2 and lzma support, but slightly
different. With a bit more macros it could maybe have used the gzip.c
USE_INFLATE code path. But I felt that the many macros didn't really help
understand the code. So the unzip routine has a slightly different code
path for ZSTD.

https://sourceware.org/bugzilla/show_bug.cgi?id=26632

Signed-off-by: Mark Wielaard <mark@klomp.org>