dracut: fix gzipped initramfs; wtf is --rsyncable?!!

- Do not depend on lz4 directly, just prefer lz4 if the bin is there.
- Fallback to gzip rather than bzip2 compression if lz4/xz are not available.
- Remove --rsyncable argument in gzipped initramfs, WTF IS IT??? IT'S NOT IN UPSTREAM.