Don't mix GPL2 and GPL3

[simonlindholm]

Currently librcd depends on a mix of (L)GPLv2 and GPLv3-licensed libraries, making distribution of binaries impossible. We need to replace those by either newly written code distributed under MPL2 or replace GPL2 code by GPL3 code or vice versa.

More concretely:

• /include/linux.h and /src/sort.c are from Linux and so are GPL2 only. It might be possible to remove comments, and rewrite macros+inline functions, to claim that the result is either non-copyrightable APIs, or fair use. Bionic does something similar. Sorting routines are simple to replace, of course.
• /libs/asmlib/* from http://www.agner.org/optimize is GPL3 only. I haven't checked how much of the library is used, but replacing it feels viable.

I think all the other GPL-licensed libraries all say "version 2 or later", so they'd work with both options.