GSoC - Mock improvements - week 8

Good news, we're merging my changes upstream. It's been a lot of changes and the code wasn't always in the best shape, so I didn't want to submit it before all major features are implemented. Mirek Suchy agreed he'll do the code review and merge the changes. Big thanks to him for that :)
I've setup a new branch, rebased it on top of current upstream and tried to revisit all my code and get rid of changes that were reverted/superseded, or are not appropriate for merging yet.  I squashed fixup commits to their originl counterparts to reduce the number of commits and changed lines.
The changes that weren't submitted are:

• C nofsync library, because Mikolaj made a more robust nosync library that is packaged separately, and therefore supersedes the bundled one.
  Link: https://github.com/kjn/nosync/
  I did the review: https://bugzilla.redhat.com/show_bug.cgi?id=1118850
  That way mock can stay noarch, which gets rid of lots of packaging issues. And also saves me a lot problems with autoconf/automake. There is no support for it yet, because I need to figure out how to make it work correctly in multilib environment.
• nofsync DNF plugin - it's an ugly DNF hack and I consider it superseded by aforementioned nosync library
• noverify plugin - it's also a DNF hack, I will make a RFE for optional verification in DNF upstream instead

Everything else was submitted including the LVM plugin.  The merging branch is not pushed on github because I frequently need to make changes by interactive rebasing and force pushing the branch each time kind of defeats the purpose of SCM.
Other than that I was mostly fixing bugs, the only new features are the possibility of specifying additional commandline options to rpmbuild, such as --rpmfcdebug with --rpmbuild-opts option and ability to override command executable paths for rpm, rpmbuild, yum, yum-builddep and dnf, in order to be able to use different version of the tools than the system-wide version.