Delete and Rename should not be able to remove an open file,
so check share's open file table first. Behaviour is now identical
to MS-DOS 6.22.
Note: Uses a new subfunction of the FreeDOS share multiplex
interrupt int2f/0x10a6 called is_file_open()
A similar patch was applied to FDPP at
https://github.com/dosemu2/fdpp/commit/691721f1
Fixes dosemu2 tests:
test_fat_ds3_share_open_rename_fcb
test_fat_ds3_share_open_rename_ds2
test_fat_ds3_share_open_delete_ds2
test_fat_ds3_share_open_delete_fcb