MFC r321686 and r330361:
Add inline functions to convert between sbintime_t and decimal time units.
Use them in some existing code that is vulnerable to roundoff errors.

MFC r264388 (by davide):
Define SBT_MAX.

MFC r267896 (by davide):
Improve r264388.

MFC note. The SBT_MAX definition already existed on stable/10, but without
the refinement from r267896. Also, consumers of SBT_MAX were not converted,
since r264388 was not merged properly.

Reviewed by: mav

Complete MFC of r270233, also unbreak the build.

Reported by: grehan

Complete MFC of r270233, also unbreak the build.

Reported by: grehan