Properly null-terminate strings in a kernel dump header

[asomers]

Summary

A version string longer than 192 bytes will cause the version field of
a dump header to overflow. strncpy doesn't null terminate it, so savecore will
print a corrupted info file. Using strlcpy fixes the bug.

Test Plan

Build a kernel with a very long version string and use it to
generate a panic, then dump core. Check that the Panic String is not embedded
at the end of the Version String line.Version String line.