Clarify hardpps() parameter name and comment

Since 32c203577a5e by phk in 1999 (Make even more of the PPSAPI
implementations generic), the "nsec" parameter of hardpps() is a time
difference and no longer a time point. Change the name to "delta_nsec"
and adjust the comment.

Remove comment about a clock tick adjustment which is no longer in the code.

Pull Request: https://github.com/freebsd/freebsd-src/pull/640
Reviewed by: imp