Shellcheck tells us "-n doesn't work with unquoted arguments. quote or
use [[ ]]."
And testing shows:
```
docker run --rm -it ubuntu bash
root@fe85ce156856:/# [ -n $DOES_NOT_EXIST ] && echo "I ran"
I ran
root@fe85ce156856:/# [ -n "$DOES_NOT_EXIST" ] && echo "I ran"
root@fe85ce156856:/#
```
Closes#5773
To avoid an explosion of jobs, extend the existing CMake tests with
ngtcp2 and quiche support. macOS was previously moved to GitHub actions,
so the non-Linux case can be dropped.
Reported by the new script 'scripts/copyright.pl'. The script has a
regex whitelist for the files that don't need copyright headers.
Removed three (mostly usesless) README files from docs/
Closes#5141
