This apply is called when the driver receives an MXP tag from an
MXP-capable client. The mudlib can process these tags and decide
whether to allow or block them.
**Arguments:**
- `tag`: The MXP tag received from the client
**Return Value:**
- Return non-zero to block/reject the tag
- Return 0 to allow the tag
MXP tags allow clients to send structured data and interact with
the MUD through clickable elements, forms, and other rich features.
MXP support must be enabled in the driver configuration with:
```
enable mxp : 1
```