Define constants for service_links_X settings

Reading the code in theme_service_links_build_link is challenging because there's stuff in there like:

switch (variable_get('service_links_style', 1)) {
      case 1:
        ...
      case 2:
        ...
      case 3:
        ...
}

It'd be easier to grok if it was:

switch (variable_get('service_links_style', SERVICE_LINKS_STYLE_TEXT)) {
      case SERVICE_LINKS_STYLE_TEXT:
        ...
      case SERVICE_LINKS_STYLE_IMAGE:
        ...
      case SERVICE_LINKS_STYLE_IMAGE_AND_TEXT:
        ...
}