libc: Fix possible overflow in binuptime().

This is an application of the kernel overflow fix from r357948 to
userspace, based on the algorithm developed by Bruce Evans. To keep
the ABI of the vds_timekeep stable, instead of adding the large_delta
member, MSB of both multipliers are added to quickly estimate the overflow.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks