[server] explicitly set postgres LC_MESSAGES variable to C on connection We parse some error messages so we need them to be in English. It's not clear to me exactly when that's not the case, but this fixes running unittest_postgres' "test_constraint_validationerror" with py.test for Sylvain.

usr/lib/python2*/*-packages/cubicweb/dataimport/
usr/lib/python2*/*-packages/cubicweb/server/
usr/lib/python2*/*-packages/cubicweb/hooks/
usr/lib/python2*/*-packages/cubicweb/sobjects/
usr/lib/python2*/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/