Switch ECDHE group default logic for bridge/relay TLS

According to the manpage, bridges use P256 for conformity and relays
use P224 for speed. But skruffy points out that we've gotten it
backwards in the code.

In this patch, we make the default P256 for everybody.

Fixes bug 9780; bugfix on 0.2.4.8-alpha.
Merge remote-tracking branch 'public/bug9780_024_v2' into maint-0.2.4