Allow cursor behaviour to be read

[stevesims]

On Acorn systems, cursor behaviour could be read using OSBYTE &ED

We should consider adding a mechanism to read the cursor behaviour and send it to MOS

NB currently there is no room in the VDP protocol for this information - we can probably use the remaining VDP protocol bit to indicate some kind of "extended info packet".

Support will be needed in MOS for this behaviour

[stevesims]

RISC OS systems included VDU variables that could be read in BASIC using the VDU statement as a function, i.e. var% = VDU var. we should look to incorporate this kind of functionality - this may extend beyond just reading cursor behaviour