Adding a prompt on the installer before overwriting the partition table

I think there should be a prompt in the installer before overwriting the
partition tables. The current behavior is, when selecting the whole
disk, to overwrite the partition table directly.

I am suggesting this because I once by mistake double pressed enter at
the «Partitioning Disks» stage, selecting my main drive but also
selecting the default partitioning option. I quickly canceled the
operation, but it had already deleted my GPT table. I was able to
recover it, but it should have asked, like it does on the «(E)dit»
option.

I would suggest modifying install.md like this (example from the amd64
version):

--- install.md 2021-06-28 15:07:22.277778218 +0200
+++ install.md.ask 2021-06-28 15:08:44.776914131 +0200
@@ -71,7 +71,7 @@
case $resp in
[wW]*)
echo -n "Setting OpenBSD MBR partition to whole $_disk..."
- fdisk -iy $_disk >/dev/null
+ fdisk -i $_disk
echo "done."
return ;;
[gG]*)
@@ -81,7 +81,7 @@
fi

echo -n "Setting OpenBSD GPT partition to whole $_disk..."
- fdisk -iy -g -b 960 $_disk >/dev/null
+ fdisk -i -g -b 960 $_disk
echo "done."
return ;;
[eE]*)