Adds support for establishing SSH sessions over an existing "ControlMaster" [unix domain] socket in proxy mode. Details of the protocol can be found here: https://github.com/openssh/openssh-portable/blob/master/PROTOCOL.mux More details about ControlMaster sockets can be found here: https://linux.die.net/man/5/ssh_config Fixes golang/go#32958 Co-authored-by: Cyrus Katrak <ckatrak@slack-corp.com> Change-Id: Ia3ae8893262f5060ed3fadfcbe97619c9659145b Reviewed-on: https://go-review.googlesource.com/c/crypto/+/733040 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Nicola Murino <nicola.murino@gmail.com> LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com>