Merge branch 'dh-group-rekey'

These changes improve rekeying after the peer initially selected a
different DH group than we proposed.  Instead of using the configured DH
group again, and causing another INVALID_KE_PAYLOAD notify, we now reuse
the previously negotiated group.  We also send the selected DH group
first in the proposals (and move proposals that don't contain the group
to the back) so that implementations that select the proposal first and
without consulting the KE payload (e.g. strongSwan when preferring the
client's proposals) will see the preferred group first.

Fixes #2526.