Add inline functions to convert between sbintime_t

Add inline functions to convert between sbintime_t

and decimal time units. Use them in some existing code that is
vulnerable to roundoff errors.

The existing constant SBT_1NS is a honeypot, luring unsuspecting folks into
writing code such as long_timeout_ns*SBT_1NS to generate the argument for a
sleep call.  The actual value of 1ns in sbt units is ~4.3, leading to a
large roundoff error giving a shorter sleep than expected when multiplying
by the trucated value of 4 in SBT_1NS.  (The evil honeypot aspect becomes
clear after you waste a whole day figuring out why your sleeps return early.)