Generic Hunk allocator

Implementation of a general purpose Hunk allocator for an easier
portability of htq2.
* General purpose Hunk driver based on malloc/free
* Hunk memory system separation in high-level and low-level subsystems.
* High-level are the same Hunk_* calls who now call their low-level
counterparts Sys_Hunk*, which are system-dependent.
* New cvar "hunk_driver", it prints which Hunk allocator is being
currently in use.
* New cvar "hunk_trace" activates printing of messages for tracing out
  the activity on the memory system, "developer" must be set to 1 for
  this to work.
* New BOOL CMake option HT_HUNKDRIVER_GENERIC, it enables/disables use
of the generic Hunk driver.
Merge branch 'hunk_alloc'