Add MFS disk image support

[probonopd]

It would be nice if one could mount/use MFS, HFS, HFS+ disk images from within Executor.
Otherwise it is hard to make use of, e.g., https://archive.org/details/softwarelibrary_mac.

[probonopd]

E.g.,

# file '/home/user/Desktop/hd2.img' 
/home/user/Desktop/hd2.img: Apple Driver Map, blocksize 512, blockcount 41820, devtype 1, devid 1, driver count 1, contains[@0x200]: Apple Partition Map, map block count 4, start block 96, block count 41692, name MacOS, type Apple_HFS, valid, allocated, in use, readable, writable, chain compatible driver, contains[@0x400]: Apple Partition Map, map block count 4, start block 1, block count 63, name Apple, type Apple_partition_map, valid, allocated, in use, readable, writable, contains[@0x600]: Apple Partition Map, map block count 4, start block 64, block count 32, name Macintosh, type Apple_Driver43, processor 68000, valid, allocated, in use, has boot info, readable, writable, pic boot code, contains[@0x800]: Apple Partition Map, map block count 4, start block 41788, block count 32, name Extra, type Apple_Free, valid, allocated, in use, readable, writable

# file '/home/user/Desktop/some.dsk' 
/home/user/Desktop/some.dsk: Macintosh MFS data (bootable) created: Thu May 17 18:16:45 1984, last backup: Thu Jun  7 18:52:15 1984, block size: 1024, number of blocks: 391, volume name: some

[probonopd]

For MFS, maybe this is helpful:
https://github.com/d235j/fusemfs

For HFS, maybe this is helpful:
https://github.com/alanswx/LinuxFuseHFS

For HFS+, maybe this is helpful:
https://github.com/0x09/hfsfuse

[autc04]

There is some HFS support there from the old days, it's lacking a proper user interface and documentation though.
Copy or link your image files into the directory ~/.executor/images, or set the MacVolumes environment variable to point to a different directory or to a single image file.

[probonopd]

set the MacVolumes environment variable to point to a different directory or to a single image file

It works for HFS! 👍 This should be documented prominently on the frontpage.

MFS would still be neat though, and HFS+ already has an issue open at #16.