[devtools] force locale to C for postgresql test clusters Avoids initdb failure with e.g. iso8859-X locale and utf-8 encoding.

[rql2sql] remove special behaviour of symmetric relation vs DISTINCT 0542a85fe667 replacing SQL OR by hooks for symmetric relations allows that. This involve a single test value change for a border case: when querying a symmetric relation without specifying the subject nor the object, you may get some duplicated result. IMO this is fine to let the user explicitly use DISTINCT or not and to remove the dedicated handling we had which didn't let any choice. Related to #3259713