make-secnet-sites: Without --pubkeys-install, write appropriate key

make-secnet-sites: Without --pubkeys-install, write appropriate key

Rather than hoping that the first thing in the list is appropriate,
write out the first *key* we come across.

This is needed because we are going to introduce into the 'pub' list
things that are not keys.  We don't want to somehow try to write to a
.conf file something corresponding to `serial' or `pkg' or `pkgf'.

The `indefault' variable will tracks whether we are in the default
pubkey group.  Right now we don't understand pubkey groups so it is
just `True'.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>