Add kali user to the vboxsf group, if ever it exists

The vboxsf group is created by the postinst script of the package
virtualbox-guest-utils. The kali user needs to be part of this group
in order to access VirtualBox's shared folders.

This change does just that. It's effective for all the Live images
(where VirtualBox guest additions are installed unconditionnally),
and for the systems installed by the Installer image where VirtualBox
was detected (and therefore VirtualBox guest additions were installed).

Ref: <https://bugs.kali.org/view.php?id=7643>

kali-config/common/includes.chroot/usr/lib/live/config/0031-kali-user-setup

@@ -21,8 +21,9 @@ configure_usergroups() {
     # adm - read access to log files
     # dialout - for serial port access
     # kaboxer - for kaboxer
+    # vboxsf - shared folders for virtualbox guest
     # wireshark - capture sessions without being root
-    kali_groups="adm dialout kaboxer wireshark"
+    kali_groups="adm dialout kaboxer vboxsf wireshark"
 
     for grp in $kali_groups; do
 	getent group $grp >/dev/null || continue

kali-config/common/includes.installer/kali-finish-install

@@ -59,8 +59,9 @@ configure_usergroups() {
     # adm - read access to log files
     # dialout - for serial access
     # kaboxer - for kaboxer
+    # vboxsf - shared folders for virtualbox guest
     # wireshark - capture sessions in wireshark
-    kali_groups="adm dialout kaboxer wireshark"
+    kali_groups="adm dialout kaboxer vboxsf wireshark"
 
     for user in $(get_user_list | grep -xv root); do
 	echo "INFO: adding user '$user' to groups '$kali_groups'"