un-,mkinitramfs: add support for zstd

Add support for zstd compression and decompression.

Always use multithreaded compression, as zstd still
generates identical archives.

The zstd cmdline tool is usually able to decompress
gzip, xz, lzma and lz4 so make sure to try the tool
early.

Signed-off-by: Norbert Lange <nolange79@gmail.com>