Fixed ALL compiler warnings

I fixed all compiler warnings from clang/gcc. If someone wants to use OpenSC in a productive environment, the least thing you want to do is to check the compiler warnings! (If you are Apple you might even want to turn on more warnings that usual... otherwise you `goto fail`).

In the process I found and fixed some critical bugs. Especially `pkcs15init_initialize` from src/pkcs11/framework-pkcs15init.c needs a rewrite as it implements a wrong prototype (see c18ec388bc29b9a084d5b1ebbaff69098f5a6ddb).

Also, we should start to agree upon some coding rules. The first, I want to add is to treat compiler warnings as errors!