A real life scenario is that cores are compressed to reduce
size of dumpon partition, but we either don't care about space
in the /var/crash or we have a filesystem level compression of
/var/crash. And we want cores to be uncompressed in /var/crash
because we'd like to instantily read them with kgdb.
In this case we want kernel to write cores compressed, but
savecore(1) write them uncompressed.
This patch doesn't support ZSTD yet, since the library isn't
yet installed in the base system and I'm reluctant to statically
compile it into savecore(1).