nntpd: move {newsgroup} name check to config

nntpd: move {newsgroup} name check to config

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