Fix compiling on linux
Fixed player locked on map start

In SP, when a map finished with a locked player (e.g. t1_rail) the
player is also locked (can't be moved) on the next map. The lock state
is also saved in the save game, so reloading the map does not fix it.
Fix some compiler warnings
Fixed #282 - SP and MP now use libpng instead of relying on hand-rolled
implementations. Also moved common dependencies into the lib/ folder. As a
result, had to tweak a lot of files!
Fixed SP project files for moved dependencies.
Unused variable in SV_ConSay_f
Remove old/wrong license header from sys_loadlib.h.
Will be replaced in a coming commit with correct global license.
Merge remote-tracking branch 'TheRikke/master'

Conflicts:
	code/zlib32/deflate.cpp