Better ping implementation.

src/d_clisrv.c

src/d_clisrv.h

src/d_netcmd.c

src/d_netcmd.h

src/djgppdos/i_video.c

src/hu_stuff.c

src/hu_stuff.h

src/m_menu.c

src/screen.c

src/screen.h

src/sdl/i_video.c

src/win32/win_vid.c