Run Clang static analyser over codebase to find dead code and remove it

It can be run like this:

```
meson setup builddir
ninja -C builddir scan-build
or
meson compile -C builddir scan-build
```

Also runtime coverage is possible:

```
meson setup builddir -Db_coverage=true
meson compile -C builddir
meson test -C builddir
ninja -C builddir coverage-html
```