The main advantage here is that this allows to get rid of the gdtoa dependency, it is also preferable to have some code here that's being maintained.
Feature-wise both are mostly identical, stb also support MSFT integer size modifiers and hexadecimal floats.
This solves two problems:
* The linked list is too slow, a map is better. A map cannot be used with statically allocated CVARs because order of initialization is undefined.
* The current CVAR system is an unordered mishmash of static variables and dynamically allocated ones and the means of identification are unsafe. With this everything is allocated on the heap so it can all be handled the same by the cleanup code.
Rationale:
1. Now to reset a CVAR to default, the user doesn't need to remember the default value.
2. If a modder wants to reset a CVAR via menu command, they don't need to keep menudef and cvarinfo in sync.
Looks I completely forgot this part when refactoring the interface - it must account for the callbacks not doing anything due to the network code not being operational yet.
