Yet another regression introduced by Kristaps when he switched from

Yet another regression introduced by Kristaps when he switched from
Berkeley DB to SQLite3: In the .In parser, the logic got inverted.
The resulting NULL pointer access was found by clang;
scan log provided by Ulrich Spoerlein <uqs at FreeBSD>.

The best fix is to simply remove the whole, pointless custom
handler function for .In and let the framework do its work.
Now searching for included header files actually works.

While here, remove the similarly pointless custom .St handler,
fix the return value of the .Fd handler and disentangle the
spaghetti in the .Nm handler.