libsystemd-bus: catch up with latest kdbus changes

The name list and query functions now work differently in kdbus, as the
result is stored in the connection's pool, rather than being copied back
through the ioctl buffer. That also safes us some logic, as we don't
have to resize the communication buffer in a loop with growing buffer
sizes.

Due to this, KDBUS_CMD_MSG_RELEASE was renamed to KDBUS_CMD_FREE, as it
is now used outside the message logic as well.