Add method to wait for time to be synchronized initially

[mcpherrinm]

With systemd, I can order targets after the time is synced: https://manpages.debian.org/testing/systemd/systemd-time-wait-sync.service.8.en.html

I would like a similar method to have scripts or other tools block until ntpd-rs has successfully synced (ie, is not stratum 16) and written the time to the kernel successfully.

At least on a system I'm testing on, systemd reported that the time was synced when the ntpd server was shut down and thus couldn't have synced. ntp-ctl status says stratum 16, and my adjtimex test tool says the clock is not synchronized. Systemd does suggest that its time-wait-sync "unreliable" with 3rd party ntp clients, so that checks out.

Indirectly, I can do this via the adjtimex syscall in a generic tool. However, I would appreciate it if this could be supported "out of the box" that allows explicitly waiting for ntpd-rs.