Merge remote-tracking branch 'origin/inboxdir'

* origin/inboxdir:
  config: remove redundant inboxdir check
  config: support "inboxdir" in addition to "mainrepo"
  examples/grok-pull.post_update_hook: use "inbox_dir"

diff --cc lib/PublicInbox/WwwStream.pm
Simple merge

diff --cc script/public-inbox-mda
Simple merge

diff --cc t/mda.t
Simple merge