Better doc integration

Better doc integration

Now some checks must pass before we generate the docs:

- .Nm and .Fo names are identical.
- All .Fn names are referenced in .Nm and .Fo.
- All functions from the headers have an .Nm reference.
- No .Nm reference documents a non-existent function.
- No .Xr reference is dead.

In practice this allowed me to catch many stale references very easily.
Since those were basicaly mechanically caugth typos, I did not update
the date nor copyright information in the affected documents.

Closes #250