* More than 80 trucks set to cross Qamishli border
* Will be first U.N. aid shipment via Turkey
* Concern aid will only reach government-controlled areas
By Dasha Afanasieva
GAZIANTEP, Turkey, March 19 (Reuters) - A U.N. convoy of about 80 trucks is ready to cross the Turkish border into Syria for the first time, aid officials said on Wednesday, in a step they hope will pave the way for...