Automatically generate symlinks for virtual categories.

[Vitaly Magerya]

And here [1] is a new version that does the same thing,
but uses INDEX file instead of traversing the ports tree.
It's quite faster this way (assuming your INDEX is up to date,
maybe it's better to allow both algorithms?).

Disclaimer: I did not test it properly.

The thing that looks strange to me is that the original script
appends main category to the name of port when symlinking.
I copied that behavior for safety, but are there really
naming conflicts in the ports tree?

[1] http://tx97.net/pub/files/auto-symlink-virtual.sh