Move gamestate handling to a new file

src/hu_stuff.c

src/netcode/Sourcefile

src/netcode/d_clisrv.c

src/netcode/d_clisrv.h

src/netcode/gamestate.c

src/netcode/gamestate.h

src/netcode/net_command.c

src/netcode/server_connection.c

src/netcode/tic_command.c