Add more systemd notifications

[zigarn]

• reloading notification, allow to switch to Type=notify-reload and remove ExecReload in the systemd unit
• stopping notification, allow systemd to know in case of stopping from non-systemd

[pajod]

I submitted a very similar patch as part of #3285 which is only slightly different:

• your code for the timestamps is both correct where mine was wrong and more obviously the correct clock
• I have an extra branch because I try to also gracefully handle the edge case of aborted master-reexec without confusing systemd about which is the new-old $MAINPID

[pajod]

Beware that systemd considers most unit parsing errors safe to proceed with, so this can unexpectedly revert to the default of simple on v<253, e.g. on some stable distributions. Debian 12 "bookworm" ships v252 and Ubuntu 22.04 LTS "jammy" ships v249.

[zigarn]

Thanks for the info, I updated the PR to explain this.

[pajod]

Lines starting with # are comments in systemd. (Please keep up expecting software to be designed with common conventions in mind! That - reasonable - expectation is what keeps is from straying further into "surprise feature" territory.)