utils: make shk2image use acmd -convert if disk image doesn't exist and acmd is available

[IvanExpert]

This way extended files in archive (e.g. .sea file) can be supported (use nulib2 -xe, then nulib2 -xa, then acmd -convert). AppleCommander, because of its Java requirement, is not installed with A2SERVER, though it is available if A2CLOUD is also installed.

It would be better still if cppo or a similar utility could write AppleDouble (or AppleSingle, MacBinary, or some other flattened format for extended files) directly into disk image files, as AppleCommander's support for doing so is limited (always creates new disk image) and has bugs (see AppleCommander issue).