Rotation RPM and step settings for HFE files (and/or adding PC-88VA, FM77AV20/40 host machines)

[Peredonov]

First of all thank you to the dev and any contributors for this amazing project.

Currently, the D88/D77 disk image files archived or generated by legacy dumping software on PC-8801 and FM-7 computers need to be converted to HFE for use with Flashfloppy. However, 2D images are set to 300rpm (250kbit/s) and single step, which works with most models, but later PC-88VA (and VA2/3) models use drives that read 2DD disks at 360rpm (300kbit/s), and later FM77AV20/40 models in turn use double step for 2D disks.

As a result, unedited HFE files for 2D images are unusable on these later models, which are often sought after for their additional capabilities and features. In the case of a PC-88VA, the images appear to function initially, but loading times are extended, saving/loading user data results in I/O errors, and there can also be graphical glitches. Issues with unedited HFE files on later FM77AV models were noted in issue #885 also.

Currently, the problem can be manually worked around using a hex editor to change the appropriate header information in the HFE files or using greaseweazle software (to change rotation speed), and HxC Emutator Software (to set double step), but a Flashfloppy setting, or addition of a corresponding host machine type, would be much more convenient, and would allow the same image to be usable across older and newer models without conversion. It would also address #932 which seems to related to an external FDD drive that also uses the faster rotation speed for 2D disks. The addition of host machines would also preempt issues when loading D88 files once support for those is added as requested in #288 (which would also be extremely convenient!).