libdb: Reads DB_CONFIG from the current working directory

Postfix before 2.11.10, 3.0.x before 3.0.10, 3.1.x before 3.1.6, and 3.2.x before 3.2.2 might allow local users to gain privileges by leveraging undocumented functionality in Berkeley DB 2.x and later, related to reading settings from DBCONFIG in the current directory...