Remove useless libc, elf and ucontext, add comments to exceptions

improve propagation of errors in raise_exception

src/kernel/mod.rs

@@ -2,7 +2,6 @@ pub mod process;
 pub mod thread;
 pub mod mgerror;
 pub mod system;
-mod ucontext;
 mod synch;
 mod thread_manager;
 pub mod exception;