Fix integer overflow detection in dd

dd(1) tried to detect whether the seek offset would overflow, but it failed
to account for the case where the provided argument was negative and the
file was a regular file (negative seeks are allowed for character devices).
I fixed it, and added a regression test.

Reported by: Coverity
CID: 1368659
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp