Refactor global variables out of doomcom

src/netcode/client_connection.c

src/netcode/d_clisrv.c

src/netcode/d_net.c

src/netcode/i_net.h

src/netcode/i_tcp.c

src/netcode/net_command.c

src/netcode/server_connection.c

src/netcode/tic_command.c