upstream: Allow some keywords to expand shell-style ${ENV}

upstream: Allow some keywords to expand shell-style ${ENV}

environment variables on the client side.  The supported keywords are
CertificateFile, ControlPath, IdentityAgent and IdentityFile, plus
LocalForward and RemoteForward when used for Unix domain socket paths.  This
would for example allow forwarding of Unix domain socket paths that change at
runtime.  bz#3140, ok djm@

OpenBSD-Commit-ID: a4a2e801fc2d4df2fe0e58f50d9c81b03822dffa