VID_SDL2/SYS_POSIX: Move initing of signal handler ...

Just some cleanup in vid_sdl2. No idea why they set signal handler twice
for each initialization of video subsystem before...

Need to look into the signal handling, current usage is deprecated and
looks flaky.
SYS_POSIX: Make InitSig for linux only until investigated
CL_INPUT: Add cl_weaponforgetorder, from BLooD_DoG ...

See:
http://www.quakeworld.nu/forum/topic/4534/ktx-forced-weapon-change-when-ammo#97236

for explanation.
CL_INPUT: Cosmetics