[kernel] Added the process_kill syscall

Added process_kill, and also cleaned up all the disparate types being
used for thread/process exit codes. (Now all int32_t.)

src/kernel/objects/process.cpp

@@ -52,7 +52,7 @@ process::create_kernel_process(page_table *pml4)
 }
 
 void
-process::exit(unsigned code)
+process::exit(int32_t code)
 {
 	// TODO: make this thread-safe
 	m_state = state::exited;