ProxyCommand as both a resolver and connector

[Igor Bukanov]

I see that the hostname canonicalization configuration options is still
rather limited. As that works on DNS level they are of not use if one has
to use ProxyCommand to connect over a proxy connection or through a common
gateway name where one uses different port numbers to connect to different
intranet names.

What would be ideal is to extend the ProxyCommand to both return the
resolved universal name for the given short name and to connect to that
universal name. For example, the proxy can first print the resolved name on
its stdout before proceeding with other data.