Change copyright in dldi and disc_io files to zlib License

[chishm]

Align the license of dldi.h, dldi.c and disc_io.h to match the rest of libnds. This is a more permissive license, and will reduce copyright issues that could (have) arise in hobby projects using libnds without reproducing the 3-Clause BSD license.

Additionally, add Tim Seidel to copyright notice in files where he has contributed code.