Only write to the pid file on boot.•••Being able to change this after first boot is error prone and does
not work well on system-wide installs where the server needs root
to write the file.
Closes #566.
Add a new runtime directory and move the pid file to it.•••The data directory is intended for persistent files whereas the pid
file is ephemeral. This distinction doesn't matter by default but
on system-wide installs ephemeral runtime files go in /var/run or
/run instead.