lDebug wants to access the environment while executing
as a device driver. Instead of building it in a variable
then copying it to 68h:0 after FDCONFIG.SYS processing,
just build it there to begin with.
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
