VDC: Creating a file with too-long a filename doesn't de-dupe

[dansanderson]

Test Environment (required)
You can use MEGA65INFO to retrieve this.

• Platform: MEGA65R6, xemu
• Core Commit: 3c10488
• ROM Release: 920409

Describe the bug
If you specify a file name with more than 16 characters in DOPEN, it will be correctly shortened to 16. However, if you do this repeatedly, a file with the same name will always be created.

To Reproduce

10 DOPEN#1,"0123456789ABCDEFGHI,P",W
20 PRINT#1,"HALLO"
30 DCLOSE#1

Expected behavior
On an IEC drive, the filename is truncated to 16 characters, and only creates the file if it does not exist, otherwise it overwrites.

On a disk image or internal floppy, the filename is truncated to 16 characters and a new file is created each time. The virtual disk controller should behave similarly to IEC.

Additional context
I don't know if this is a CBDOS issue or a VDC issue.