With 50K newsgroups in the config file, this doesn't slow down
`PublicInbox::Config->new->fill_all' any measurable amount on
my busy old workstation.
This should prevent invalid newsgroup names from getting into
into extindex and catch user errors sooner, rather than later.
v2:
- delete {newsgroup} if invalid to avoid ->nntp_url link
- simplify -imapd and explain remaining check