Improve syscall definitions

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

src/drivers/nulldrv/main.cpp

@@ -6,6 +6,7 @@ extern "C" {
 	int32_t fork();
 	void sleep(uint64_t til);
 	void debug();
+	void message(const char *msg);
 
 	int main(int, const char **);
 }
@@ -15,6 +16,7 @@ int
 main(int argc, const char **argv)
 {
 	int32_t pid = getpid();
+	message("hello from nulldrv!");
 	//int32_t child = fork();
 	//debug();
 	for (int i = 1; i < 5; ++i)