Support compressed crash dumps in crashinfo(8).

Temporarily decompress a copy of a crash dump compressed with either
gzip or zstd and run various tools against the decompressed copy while
generating the crash information. The uncompressed copy is deleted when
the script exits.

Note that crashinfo is enabled by default, so this will attempt to
decompress the most recent compressed crash dump after a crash that
generates a compressed crash dump. Users who wish to only do offline
analysis of compressed crash dumps can disable crashinfo in rc.conf.

Tested by: ler
Reviewed by: markj
MFC after: 2 weeks