Implement support for freestanding package names in recipes.

* Package names used to be linked to port names (they could only extend
  it). Now, independent package names and versions can be set by means of
  PACKAGE_NAME and PACKAGE_VERSION recipe attributes.
Fix #32: crash when cleaning up after removed recipe.

* It is possible that the dict entry for the port has already been
  removed, so check that it's indeed there.
Fix #3: broken networking in chroot.

* Haikuporter now copies the network settings directory into the chroot,
  such that networking should work just as usual.