harare zimbabwe electoral commission zec chairwoman rita makarau resign friday months vote whose credibility crucial new army-backed government efforts re-engage international lenders lure investors senior government source say makarau see ally 93-year-old former president robert mugabe give reason resignation official decline name say president emmerson mnangagwa swear two weeks ago wake de facto military coup end mugabe 37-year rule pledge hold elections schedule next year finance minister patrick chinamasa mention credibility elections least five time budget speech thursday sign vote importance shore harare democratic legitimacy opposition party demand reform electoral system say skew rule zanu-pf party favor makarau accuse partisan oversee overhaul voters roll opposition movement democratic change describe shambolic makarau answer mobile phone spokesman zec unable confirm makarau departure head zimbabwe election commission quit source say