When dumping INSERT statements, optionally add ON CONFLICT DO NOTHING.

Author: Surafel Temesgen
Reviewed-by: Takeshi Ideriha, Nico Williams, Dilip Kumar
Discussion: https://postgr.es/m/CALAY4q-PQ9cOEzs2%2BQHK5ObfF_4QbmBaYXbZx6BGGN66Q-n8FA%40mail.gmail.com