Automatically generate symlinks for virtual categories.

[Vitaly Magerya]

Some random comments:
- you really want to add usage [1] ("-d" is intentionally not documented)
- when destdir does not exist the script should make one [2]
- you really want to handle whitespace in paths [3]
- there's no easy way to use an INDEX that is not under portsdir;
  I think that in -i you should specify a path, not a filename [4]

(The patches should be applied in the listed order).

[1] http://tx97.net/pub/patches/auto-symlink-virtual-usage.diff
[2] http://tx97.net/pub/patches/auto-symlink-virtual-destdir.diff
[3] http://tx97.net/pub/patches/auto-symlink-virtual-whitespace.diff
[4] http://tx97.net/pub/patches/auto-symlink-virtual-index.diff