Kernel+LibC: Add a dbgputch() syscall and use it for userspace dbgprintf().

The "stddbg" stream was a cute idea but we never ended up using it in
practice, so let's simplify this and implement userspace dbgprintf() on top
of a simple dbgputch() syscall instead.

This makes debugging LibC startup a little bit easier. :^)

Kernel/Process.cpp

@@ -2716,3 +2716,9 @@ int Process::sys$dump_backtrace()
     dump_backtrace();
     return 0;
 }
+
+int Process::sys$dbgputch(u8 ch)
+{
+    IO::out8(0xe9, ch);
+    return 0;
+}