Add release-candidate-checklist make target

This attempts to document and automate a checklist for producing a
release candidate, to the extent that this is possible.
Add dependency on release and doc builds
Update manual checklist items
Run test suite as part of checklist
Run examples and benchmarks as part of checklist
Make source-dist tarballs
Remove examples check (already part of tests)
Add suggestions from @staticfloat
Add check if checklist produced dirty repo state
Replace hard-coded make with $(MAKE) variable
Bump juliadoc so that we run the proper version of julia when running doctests
Add a few more steps regarding packaging
Run DocCheck to detect undocumented items
Make target: release-candidate-checklist -> release-candidate
Add a few more steps
We can't run source-dist if we haven't bumped VERSION yet.  ;)
Make these "negative steps" fail-early options
Keno's recent backtraces work has added backtraces for these errors!
We had the wrong condition here
Looks like helpbd.jl has some new syntax rules