Grab dependency name & version strings as argv-style arrays

Grab dependency name & version strings as argv-style arrays
- Related to ticket #22, this fixes rpmdsMerge() on data grabbed from
  header. Prior to this, the following would blow up with memory corruption
  as headerGet() string array wasn't NULL-terminated
	rpmds d1 = rpmdsNew(h, RPMTAG_REQUIRENAME, 0);
	rpmds d2 = rpmdsSingle(RPMTAG_REQUIRENAME, "foo", "1", 0);
	rpmdsMerge(&d1, d2);