Prevent the Gnus registry from being loaded twice at startup

* lisp/gnus/gnus-registry.el (gnus-registry-load): Check if the
  registry is already loaded, and don't load again unless new optional
  FORCE argument is non-nil.
  (gnus-registry-clear): New function to clear the registry, added as
  a Gnus shutdown. Now that loading doesn't unilaterally reset the
  registry, we need to make sure it is reloaded when Gnus is.
1 job for master in 53 minutes (queued for 1 second)
Status Job ID Name Coverage
  Test
failed #2242
test-all

00:53:00

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:957: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1