rationalize library header name indexing

[zygoloid]

Right now, we index library headers something like this:

\indextext{\idxhdr{<blah>}}%
\indexlibrary{\idxhdr{<blah>>}}%

We should provide a dedicated macro for this (perhaps \indexhdr{blah}). Once that's done, we should consider removing these from the index of library names, since they're not library names in the normal sense. (Maybe a separate 1-page index of library headers would be useful, though.)

[jensmaurer]

Now moving all library headers from the "index of library names" into a new index. Library headers also remain in the main index.