Two new configuration options are added:
     - "enable_ipv6" to enable/disable the overall use of IPv6
     - "ipv6_server" to enable/disable the use of IPv6 sockets when running
       a server (when "enable_ipv6" is enabled)

The previous code did not work when the list did not start at index 0
of the inventory list (issue #779).

The 50 ms delay causes frequent test failures on certain systems with
slow network stacks, these failures go away with the 100 ms variant.

Play player_damage.ogg when recieving damage and additionally play player_falling_damage.ogg when recieving falling damage

This reverts commit 649dca4d.