Net struct init

Do a check for valid num_files before doing
assert(!m_mesh_update_thread.IsRunning());

In particular, don't execute the assertion if all media has already
been received.

Also write something about rebuilding shaders to infostream in
Client::afterContentReceived()

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)

Check in all 6 directions instead of just 3.

Spawn single particles or make use of ParticleSpawner for many randomly spawned particles.
Accessible in Lua using minetest.spawn_particle and minetest.add_particlespawner.
Increase Protocol Version to 17.

Conflicts:
	src/clientserver.h