enumerator: Add venumerate() method to enumerator_t that takes a va_list

enumerator: Add venumerate() method to enumerator_t that takes a va_list

This will allow us to implement e.g. enumerator_cleaner without having to
use that unportable 5 pointer forwarding or having to define a callback for
each instance.

A generic implementation for enumerate() is provided so only venumerate()
has to be implemented, which may be simplified by using the VA_ARGS_VGET()
macro.