- cleanup data type and use guint32 consistently. We might want to
  introduce a new "infinity" value. But since libnm's
  NM_SETTING_IP_CONFIG_DHCP_TIMEOUT asserts against the range
  0 - G_MAXINT32, we cannot express it as -1 anyway. So, infinity
  will have the numerical value G_MAXINT32, hence guint32 is just
  fine.

- make use of existing ipv6.dhcp-timeout setting and add global
  default configuration in NetworkManager.conf

- instead of having subclasses call nm_device_set_dhcp_timeout(),
  add a virtual function get_dhcp_timeout().