BuildSystem: Cleanup userspace directory layout

userspace programs are now in userspace/programs
userspace tests are now in userspace/tests

This makes listing userspace projects much cleaner. Libraries were
already separated to their own directory, so other programs should also.

userspace/programs/whoami/main.cpp

@@ -0,0 +1,14 @@
+#include <stdio.h>
+#include <unistd.h>
+
+int main()
+{
+	char* login = getlogin();
+	if (login == nullptr)
+	{
+		printf("unknown user %d\n", geteuid());
+		return 1;
+	}
+	printf("%s\n", login);
+	return 0;
+}