savecore: add an option to save a live minidump

The new '-L' flag will cause savecore to invoke the new mem(4) kernel
dump ioctl, taking a dump of the running system and writing the result
to a temporary file. Validation of the dump header is performed, similar
to regular crash dumps, and the final result is written to
livecore.X[.zst|.gz].

Also added is the '-Z' flag, which instructs the kernel to compress the
livedump compressed with zstd, akin to the existing -z flag. This option
has no effect in normal savecore(8) operation, but in theory could be
extended to perform such compression while reading the dump from the
dump device.

Encryption is unsupported for live dumps.

For example: 'savecore -Lz /var/crash' would create:
/var/crash/livecore.0.gz

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34347

(cherry picked from commit 0a5c04a8926e0418c556dfb65efbb24b3bf41dd0)