[SP] don't use _exit() because it breaks gprof
[MP] don't print an error message on an empty command string, as it did before in vanilla src.  this happens a lot at beginning of demos.
[MP] use exit() instead of _exit() for gprof
[MP] correct fix for gcc segfault issues from SSE instructions.  actual issue was misaligned stack, and running mods compiled with ancient gcc versions.
[MP] video recording fixes
 - skip first frame because it's the loading screen (?)
 - don't record audio frames if we aren't recording video frames
 - remove 2gb filesize limit on output.  might need to make this a cvar, since it's still useful, but breaks using pipes for output on linux.
[MP] fixes infinite loop in demo screen
[MP] if delta frame is invalid, fall back to a valid one.  the result is slightly
better than just freezing, most of the time nothing significant is lost.
[MP] dont display the profile menu while playing a demo, since the player can't
'join the game' anyway
[MP] make it compiling on visual studio again
[MP] strip . and : from player name too, since FS_CreatePath refuses to make
those files if those chars are doubled
[MP] remove unused variable