Implement suspend/hibernate handling on Solaris/illumos for `upsmon`

[jimklimov]

Follow-up from #2597 and #1070.

With the work above done, we have a sort of generic handling for clock jumps, to avoid rash decisions about stale UPS data causing FSD. Still, it may be nice to integrate with OS facilities directly.

[jimklimov]

Note: per FOSDEM discussion with @ptribble the SIGTSTP/SIGCONT handling can be an answer to this puzzle (and relatively portable to other POSIX systems): if a signal handler is registered, the OS waits until the process returns from handling. This allows us all the time in the world (ideally no too much of it) needed to handle going into hibernation (flip a flag when process is told to stop?) and waking up (process the flag and/or clock jump when process is told to continue?)

Most of the actual code around sleep/wake-up should already be here thanks to #2597.