Add '-u' switch that would uncompress cores that were compressed by
kernel during dump time.

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.

Reviewed by: markj, gallatin
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D27245