CPM Disc access routines

These routines are a port of the Hitech C routines.

They've been modified to work within the z88dk stdio framework and
seem to quite well within the tests that I've made. There is no distinction
between binary and text files.

May 2020 - feilipu. Added sequential read and write to try to improve performance for large buffer moves. Issue uncovered through performance comparision with CP/M PIP tool.

