man pages should be generated for binaries

[r0ml]

An installation from the generic unix tarball doesn't install man pages for alex and happy, despite these existing in the sources.

There is also no man page for cabal install, but I don't think one has been written, so this is probably less important.

[r0ml]

We'll need to extend Cabal for support for building and installing man pages (and preferably other forms of docs too).