Improve dsktool portability and fix some issues

[Robert-van-Engelen]

This PR improves dsktool portability and fixes some issues:

• update dsktool/Makefile to compile dsktool on Linux and MacOS without warnings and errors
• update dsktool/dsktool.c to remove deprecated malloc.h since malloc/free are declared in stdlib.h
• update dsktool/dsktool.c to fix char signedness issues (char may or may not be signed, no assumptions should be made)

I've tested dsktool on Linux and MacOS using a 720K MSX .dsk image created with WebMSX for my ForthMSX project.

I've not tested on Windows. However, I do not expect problems, because these source code changes are minimal and follow the C/C++ standard.