Add interactive abort check feature to checkisomd5 (#555107)

[Radek Vykydal <rvykydal redhat com>]

This series of patches builds on patch by Darryl L. Pierce, already
reviewed with some comments here:
https://www.redhat.com/archives/anaconda-devel-list/2010-January/msg00039.html
and adresses some of the comments. ([3/5] is independent fix)

* Uses symbolic constants for return values (as Chris suggested).

* Uses Esc instead of Enter to abort check.

* Fixes exit value of checkisomd5 command to fail (return 1 instead
  of 0) if md5 sum was not found. I think this is ok for clients
  of the command - livecd-tools and liveusb-creator - which stop
  and ask in case of fail. This behavior is also what man page has
  been saying.

* Abort of check means also fail of checkisomd5 which is ok wrt
  livecd-tools and liveusb-creator 

* Adapts anaconda to callback prototype change.

I will need to coordinate pushing of isomd5sum change with anaconda changes so
that anaconda is buildable. I have no idea about when and how to do it.
There is also fedora clone of BZ for the reference - #546412