libmetrics: linux: update_ifdata cleanup
libmetrics: linux: update_ifdata cleanup

use strtoul for consistency, add some comments and read returned valued to
avoid the following warning :

  metrics.c:242: warning: ignoring return value of 'strtol', declared with attribute warn_unused_result


git-svn-id: 27e0aca8c7a52a9ae65dfba2e16879604119af8c@1473 93a4e39c-3214-0410-bb16-828d8e3bcd0f

fastly/ganglia,fastly/ganglia,fastly/ganglia,fastly/ganglia,fastly/ganglia