Kernel+LibC: Turn errno codes into a strongly typed enum

..and allow implicit creation of KResult and KResultOr from ErrnoCode.
This means that kernel functions that return those types can finally
do "return EINVAL;" and it will just work.

There's a handful of functions that still deal with signed integers
that should be converted to return KResults.

Kernel/Devices/PS2MouseDevice.cpp

@@ -288,7 +288,7 @@ KResultOr<size_t> PS2MouseDevice::read(FileDescription&, size_t, UserOrKernelBuf
 #endif
         size_t bytes_read_from_packet = min(remaining_space_in_buffer, sizeof(MousePacket));
         if (!buffer.write(&packet, nread, bytes_read_from_packet))
-            return KResult(-EFAULT);
+            return EFAULT;
         nread += bytes_read_from_packet;
         remaining_space_in_buffer -= bytes_read_from_packet;