Automatically generate symlinks for virtual categories.

[Vitaly Magerya]

> I'll added your script to mine as a option.
Yeah, I've done that too in the mean time, take a look: [1].
In that version both traverse algorithms share common linking code,
it seems more maintainable this way.
(I've added -w and -i to catch up with you, but the code overall
is quite different, sorry about that).

There's a question about the test for main category dir:
if you use -w to specify a directory different than that of -p,
a simlink "$whereto/category/portname-category" will be created.
Maybe "$whereto/category/portname" would be the right thing in this case?

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