lastcomm: improve timestamp display

Adjust the lastcomm command to output timestamps with a precision of
seconds.

Reported by: Dr. Andreas Longwitz
Reviewed by: emaste
Relnotes: Yes
Sponsored by: DSS Gmbh
Pull Request: https://github.com/freebsd/freebsd-src/pull/802

(cherry picked from commit 6f4ce7e89ba4fa52566704533920352aa65c7f9b)