Proxy command formatting now recognises `%proxyhost' and

Proxy command formatting now recognises `%proxyhost' and
`%proxyport' in addition to the other things it interpolates. This
is useful when using the Unix-only `Local' proxy type: it allows me
to, for example, set up a proxy command such as
  ssh %proxyhost nc -q0 %host %port
and then enter the name of the proxy machine in `Proxy hostname',
which makes it marginally more convenient to quickly change to using
a different proxy to get at the same target.

I haven't documented this, because we currently don't document the
Local proxy type at all. Possibly we should, though.

[originally from svn r5073]