Improve syscall definitions

- Allow constant id specification
- Define function signature in SYSCALL macro
- Move implementation into src/kernel/syscalls/*.cpp

src/kernel/syscalls/message.cpp

@@ -0,0 +1,14 @@
+#include "log.h"
+#include "scheduler.h"
+
+namespace syscalls {
+
+void
+message(const char *message)
+{
+	auto &s = scheduler::get();
+	auto *p = s.current();
+	log::info(logs::syscall, "Message[%d]: %s", p->pid, message);
+}
+
+} // namespace syscalls