Fill in the haddock-interfaces and haddock-html fields in the
package.conf files.

To do this I had to make some changes:

  - haddock-interfaces requires the value of $(datadir).  We can't
    just plug this in, because $(datadir) might change at install-time
    (eg. a Windows installer can be placed anywhere, as can a Unix
    binary .tar.gz distribution).  The current trick is for the
    compiler to splice in the value of $libdir in package.conf at
    runtime.  So we could extend this mechanism and tell the compiler
    the value of $datadir via a command-line option, but that seems
    ugly.

    On Windows, $datadir==$libdir, so we don't need any changes:
    package.conf still uses $libdir, and a Windows installation is
    independent of its absolute location.  Even 'make install' on
    Windows should have this property.

    On Unix:
	- for 'make install' and in-place execution, we just use
          absolute paths in package.conf

	- for a binary dist, we generate a package.conf that refers
	  to $libdir and $datadir, and splice in the values at
	  install-time (distrib/Makefile-bin.in).

  - Also, I renamed $libdir to $topdir to more closely reflect its
    actual meaning.  This is somewhat malicious in that it will flush
    out all those clients using $libdir when they really shouldn't
    be :-)