Rpm package v2

I was going over some old files at the end of the year and noticed that I never sent these patches out.  This v2 branch was based off a later (most recent) master tip and tested on RHEL7.

The first rpm commit packages up files built from the source tree in their natural build locations.  When I went to extend the packaging to include a source rpm, I found that I needed a tarball of files with specific path prefix and a more precise spec file to drive the second generation build (ie, building from the generated source rpm).  So there are hokey sed and git archive tricks to this end.

I imagine Red Hat has some larger infrastructure behind their rpm generation, perhaps upstream source and packaging scripts in sub-repos, but I didn't think it was polite to rip up the entire mergerfs build just to add rpm.  RPM is horribly documented anyway and life is too short to learn any more of it this year.