Improve parallel start of OpenCPN and AvNav #13
Description
There is an issue for AvNav (wellenvogel/avnav#302) that reports problems if AvNav and OpenCPN are running in parallel.
By defining an own /tmp dir for AvNav (PrivateTmp=yes in the systemd configuration for AvNav) this was solved.
So we should add this to the avnav installer.