kpatch-build: add KVER and KREL variables

Add KVER and KREL variables, and use them where appropriate.  Also
remove the setting of ARCHVERSION in the '-s' case, since it's not
actually used anywhere in that case.
kpatch-build: download fedora src rpms from koji

yumdownloader is problematic because it doesn't allow you to download
anything but the latest released kernel.  It can also be slow at times.
Instead, for Fedora, download the RPMs from koji.