New pluggable function for freeing snprintf buffers, default constructors

* Because the allocator used to allocate memory in snprintf_fn may be
  different from the one used in alloc_fn, allow a special free function
  for snprintf buffers.
  Signature is same as free_fn, so user may just reuse that as is the case
  in the default pluggable functions.
* Add default constructors for sslhaf_cfg_t, which use default pluggable
  functions