Redmine #5627: Deal with even more extra lines from "yum check-update"

yum likes to put extra lines interspersed with the actual information
we're interested in, which is the available updates. This patch fixes
a couple of cases:

  1. Do not combine multiple lines unless all subsequent lines after
     the first start with a space.
  2. Make sure we match a '.' in the first field in the package match
     so that we don't match superfluous lines like "Obsoleting
     Packages" or "Loading mirror....".
  3. Make sure the package name is at the very start of the line. yum
     sometimes puts already installed packages in the list with an
     indentation, under the "Obsoleting Packages" header.