vpnmain.cgi: unique_subject = yes in index.txt.attr for first attempt with fresh...

vpnmain.cgi: unique_subject = yes in index.txt.attr for first attempt with fresh install

- The patches for Bug#13138
   https://patchwork.ipfire.org/project/ipfire/patch/20230603140541.13834-1-adolf.belka@ipfire.org/
   https://patchwork.ipfire.org/project/ipfire/patch/20230606104050.8290-1-adolf.belka@ipfire.org/
   work for an update to Core Update 175 but a fresh install of CU175 will still fail with
   the error when creating the root/host certificate set for the first time.
- This patch ensures that the unique_subject = yes line is addeed to index.txt.attr
   when the root/host certificate set is attempted to be created or is uploaded also for
   the first attempt.

Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>