eagle: new package

Eagle is a schematic capture and PCB layout program from CadSoft. This
is proprietary software; CadSoft provide a self-extracting shell script
with embedded tarball of the prebuilt application.

Add the latest Eagle version, 6.4.0.

I've added a small LD_PRELOAD library that redirects operations on the
license file from <eagle_install_path>/bin/eagle.key to
$HOME/.eagle.key. Without this Eagle will never get past the license
dialog (because you cannot write to the nix store).

Eagle also has issues copying its example projects to other locations;
it seems that it wants to preserve the read-only permissions from the
source over to the destination. Because of this it cannot complete the
copy operation because it cannot write the project files into to the
(read-only) project directory it just created. So wrap chmod by OR'ing
in the write-by-owner bit.