Fix handling of IPv6 dynamic forwardings.

Fix handling of IPv6 dynamic forwardings.

During the Conf revamp, I changed the internal representation of
dynamic forwardings so that they were stored as the conceptually
sensible L12345=D rather than the old D12345, and added compensation
code to translate to the latter form for backwards-compatible data
storage and for OpenSSH-harmonised GUI display. Unfortunately I forgot
that keys in the forwarding data can also prefix the L/R with a
character indicating IPv4/IPv6, and my translations didn't take
account of that possibility. Fix them.

git-svn-id: http://svn.tartarus.org/sgt/putty@10031 cda61777-01e9-0310-a592-d414129be87e