Organize system calls

* syscalls should all return j6_status_t now
* syscalls are grouped by category in name as well as in files

src/kernel/syscall.cpp

@@ -14,10 +14,6 @@ extern "C" {
 
 namespace syscalls {
 
-
-void send() {}
-void receive() {}
-
 } // namespace syscalls
 
 uintptr_t syscall_registry[static_cast<unsigned>(syscall::MAX)];