Enable date and time settings for remote files

[nanshiki]

int 21h ax=5701h updates the sft's file_time.
The file close packet adds the date/time information and sends the file_time to the server, which then sets the file's date/time information.
With this modification, the date and time information will also be copied when copying a file.

[nanshiki]

Fixed the server to accept file close packets without date and time data.

Fix to prevent date/time settings from being applied when closing files if file writing occurs.
This addresses an issue where the date and time are not updated when opening and writing to an existing file.

[nanshiki]

In the sample program for UNDOCUMENTED DOS, SFT's rel_sector/abs_sector are not initialized, and it appears that their contents are neither referenced nor overwritten in actual DOS environments. Therefore, we store the file open date/time information in rel_sector/abs_sector, compare it upon closing, and if any changes are detected, we send the data to the server for updating.

[jrohel]

Thank you for your PRs. I’ll review them later. I currently have a lot of other work.

Note:
I also noticed your fork some time ago. I have a question about it, but I’ll leave it for later when I have more time.