db: Add support for key-value pair DSNs in postgresql

These are simple space-separated key-value pair sets of options instead of the
URI style DSNs, but they are also much more flexible allowing the user to
specify client SSL certificates, server certificates, compression and
encryption levels, and much more (see [1] for more information)

[1]: https://www.postgresql.org/docs/9.1/libpq-connect.html

Changelog-Added: db: Added support for key-value DSNs for postgresql, allowing for a wider variety of configurations and environments.

wallet/db_postgres.c

@@ -17,7 +17,24 @@
 
 static bool db_postgres_setup(struct db *db)
 {
+	size_t prefix_len = strlen("postgres://");
+
+	/* We attempt to parse the connection string without the `postgres://`
+	prefix first, so we can correctly handle the key-value-pair style of
+	DSN that postgresql supports. If that fails we try with the full
+	string, which matches the `scheme://user:password@host:port/dbname`
+	style of DSNs. The call to `PQconninfoParse` here is just to verify
+	`PQconnectdb` would be able to parse it correctly, that's why the
+	result is discarded again immediately. */
+	PQconninfoOption *info =
+	    PQconninfoParse(db->filename + prefix_len, NULL);
+
+	if (info != NULL) {
+		PQconninfoFree(info);
+		db->conn = PQconnectdb(db->filename + prefix_len);
+	} else {
 		db->conn = PQconnectdb(db->filename);
+	}
 
 	if (PQstatus(db->conn) != CONNECTION_OK) {
 		db->error = tal_fmt(db, "Could not connect to %s: %s", db->filename, PQerrorMessage(db->conn));