add function to set up libpq environment

Instead of parsing the DSN for each external libpq program we
are going to execute, provide a function that feeds them all
necessary parameters through the environment.

osm2pgsql is the first user.

test/python/test_db_connection.py

@@ -3,7 +3,7 @@ Tests for specialised conenction and cursor classes.
 """
 import pytest
 
-from nominatim.db.connection import connect
+from nominatim.db.connection import connect, get_pg_env
 
 @pytest.fixture
 def db(temp_db):
@@ -48,3 +48,24 @@ def test_cursor_scalar_many_rows(db):
     with db.cursor() as cur:
         with pytest.raises(RuntimeError):
             cur.scalar('SELECT * FROM pg_tables')
+
+
+def test_get_pg_env_add_variable(monkeypatch):
+    monkeypatch.delenv('PGPASSWORD', raising=False)
+    env = get_pg_env('user=fooF')
+
+    assert env['PGUSER'] == 'fooF'
+    assert 'PGPASSWORD' not in env
+
+
+def test_get_pg_env_overwrite_variable(monkeypatch):
+    monkeypatch.setenv('PGUSER', 'some default')
+    env = get_pg_env('user=overwriter')
+
+    assert env['PGUSER'] == 'overwriter'
+
+
+def test_get_pg_env_ignore_unknown():
+    env = get_pg_env('tty=stuff', base_env={})
+
+    assert env == {}