Fix garbage value in socketpair util function, stdint?

Fix garbage value in socketpair util function, stdint?

* Fixed an issue with evutil_ersatz_socketpair_, listen_addr could all
  be compared against with agarbage values. So just memset it before
  using it anywhere.

* Nick might punch me in the face, but if we have stdint.h; (as in
  EVENT__HAVE_STDINT_H is defined), might as well use those instead of
  the manual [U]INT[X}_MAX/MIN muck in there now.