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/Syscall.cpp

@@ -70,6 +70,8 @@ static u32 handle(RegisterDump& regs, u32 function, u32 arg1, u32 arg2, u32 arg3
     case Syscall::SC_putch:
         Console::the().put_char(arg1 & 0xff);
         break;
+    case Syscall::SC_dbgputch:
+        return current->process().sys$dbgputch((u8)arg1);
     case Syscall::SC_sleep:
         return current->process().sys$sleep((unsigned)arg1);
     case Syscall::SC_usleep: