tempfork: temporarily fork gliderlabs/ssh and x/crypto/ssh

While we rearrange/upstream things.

gliderlabs/ssh is forked into tempfork from our prior fork
at https://github.com/tailscale/ssh/commit/be8b7add4057ef5a8e458b42331a7633c06d026a

x/crypto/ssh OTOH is forked at
https://github.com/tailscale/golang-x-crypto because it was gnarlier
to vendor with various internal packages, etc.
Its git history shows where it starts (2c7772ba30643b7a2026cbea938420dce7c6384d).

Updates #3802

Change-Id: I546e5cdf831cfc030a6c42557c0ad2c58766c65f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh.go

@@ -29,13 +29,13 @@ import (
 	"sync"
 	"time"
 
-	"github.com/tailscale/ssh"
 	"inet.af/netaddr"
 	"tailscale.com/envknob"
 	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/logtail/backoff"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tempfork/gliderlabs/ssh"
 	"tailscale.com/types/logger"
 )