make SET KEYBOARD= case insensitive and eliminate parameter check in MOS.

[lennart-benschop]

The MOS SET command does a case sensitive compare of its parameter KEYBOARD. While the MOS commeand names themselves and file names are case-insensitive, the KEYBOARD parameter is case-sensitive.

It also does a compare of the value. More keyboard layouts could be added to the VDP code (plus underlying vpd-gl) and the VDU handler already checks the parameter. It's not a good idea that you have to change the check in MOS, each time you extend the range of supported keyboards in VDP.

[leighbb]

As part of #60, I have added strcasecmp(). This would enable this enhancement to be easily done.

[breakintoprogram]

@lennart-benschop I will keep the check in MOS as it is useful to report back to the user when an invalid set is selected, and is no big deal to update. Will fix the case sensitivity.