optionally create a systemd service file

[DerDakon]

This is not installed so packagers can decide in which place they prefer to have it installed.

This should go in before 1.08, so either #76 could also be included, or at least simplify it's packaging after the release.

[mbhangui]

For raspberry pi kind of systems which do not have a built in RTC, a problem occurs where the timestamp entries in the log files are totally wrong. I used to have the qmail-send started after the board syncs the time from a NTP source. I don't remember now what I had exactly done and also not sure if anyone would be using qmail on such a system. Just mentioning it because the abrupt change in the times affect the qmailanalog reports. Maybe in the future this could be required to have qmail started only after the time has been set right.
https://www.freedesktop.org/software/systemd/man/systemd-time-wait-sync.service.html

[DerDakon]

This generally makes sense, but I personally would leave that as a responsibility of the local admin: if the system needs special workarounds then the admin must know them and how to fix them.

I thought for a moment that it could be useful to also depend on remote-fs.target in case someone has the home directory on a network share. But again, the admin needs to care.

[schmonz]

Otherwise looks fine.