PulleyBack: Simplify code fetching functions from DLL

PulleyBack: Simplify code fetching functions from DLL

 - First templated code in SteamWorks
 - This allows a one-liner to grab the function pointer
   with dlfunc() and store it in the correct place,
   with validity being checked and set with each function;
   if any one fails, the destructor (guaranteed to be
   called) will reset all of the function pointers.

This saves a complicated test-set-and-cleanup construction
for each function pointer.