mail_index_ext_register(): Allow registering an extension without record or header. It can still be used to register expunge/sync handlers.
mail_index_ext_register(): Allow registering an extension without record or
header. It can still be used to register expunge/sync handlers.

--HG--
branch : HEAD

jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jwm/dovecot-notmuch