Adding support for P2000C/P2012 5.25" DS/DD images

[ifilot]

Philips P2000C (P2012 model) are not yet supported by CPMtools. This update adds an entry to diskdefs to provide support.

To illustrate, the following command reads a disk image in the P2000C format.

cpmls -f p2000c disk01.img

I have tested the updated diskdefs file on a number of Philips P2000C images obtained from this source with success.