wg-installer: delete old interfaces

Add "wg_check_interfaces" and specify a timeout in the config file.
This allows to delete not used wireguard-interfaces automatically.

For example a cronjob can be installed that calls:
  . /usr/share/wginstaller/wg_functions.sh && wg_check_interfaces

Signed-off-by: Nick Hainke <vincent@systemli.org>