Old kvm_mkdb had a possibility to check old kernel's version
string retained at a hole between text segment and data segment.
It might happen a misunderstanding that the running kernel's version
matches kvm.db's version.
This had been caused by checking current kernel's version string
via /dev/kmem. The version string can get via sysctl, so use it!
already match the running kernel. Fixes an inconsistency
where /var/db/kvm_filename.db would be created, but only
if /var/db/kvm_netbsd.db didn't match the running kernel.
