savecore: add support for gzip kernel dumps

[markj]

Summary

Most of the changes are little touchups:

• Add a subroutine, compare_magic, for comparing the magic field of a dump header.
• We now use the dumpextent field of the trailing dump header to locate the leading dump header.
• DoRegularFile needs to do reads in multiples of the device's sector size, but with compressed dumps we may need to do a read that doesn't end on a sector boundary.

[cem]

sbin/savecore/savecore.c

707

I don't have operator precedence memorized for these ones. Can we add a pair of parentheses around (compress || isgzipdump)?

[markj]

• - use parens to make an expression easier to parse

[cem]

Looks good to me.

[bdrewery]

sbin/savecore/savecore.c

563

sizeof()

593

sizeof()