Kernel: Make pseudo terminals not overwrite old data

If pseudo terminal buffer was filled, old implementation would overwrite
old data. This is bad if producer is capable of producing more data than
consumer can handle.

kernel/kernel/Terminal/TTY.cpp

@@ -327,11 +327,12 @@ namespace Kernel
 		}
 	}
 
-	void TTY::putchar(uint8_t ch)
+	bool TTY::putchar(uint8_t ch)
 	{
 		SpinLockGuard _(m_write_lock);
 		if (m_tty_ctrl.draw_graphics)
-			putchar_impl(ch);
+			return putchar_impl(ch);
+		return true;
 	}
 
 	BAN::ErrorOr<size_t> TTY::read_impl(off_t, BAN::ByteSpan buffer)
@@ -365,10 +366,12 @@ namespace Kernel
 	BAN::ErrorOr<size_t> TTY::write_impl(off_t, BAN::ConstByteSpan buffer)
 	{
 		SpinLockGuard _(m_write_lock);
-		for (size_t i = 0; i < buffer.size(); i++)
-			putchar(buffer[i]);
+		size_t written = 0;
+		for (; written < buffer.size(); written++)
+			if (!putchar(buffer[written]))
+				break;
 		update_cursor();
-		return buffer.size();
+		return written;
 	}
 
 	void TTY::putchar_current(uint8_t ch)