[client] Deduplicate SSH client handshake and bound it with a deadline

Extract the dial-then-handshake sequence into nbssh.Handshake, which
applies the context deadline to the socket for the duration of the
handshake. Previously only the Android client did this; the CLI, wasm
and SSH proxy paths could block forever on a peer that accepts the TCP
connection and then goes silent, since ClientConfig.Timeout is not used
by NewClientConn.

client/ssh/handshake.go

@@ -0,0 +1,45 @@
+package ssh
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net"
+	"time"
+
+	log "github.com/sirupsen/logrus"
+	"golang.org/x/crypto/ssh"
+)
+
+// Handshake runs the SSH client handshake on an already dialed conn and
+// returns the resulting client. Dialing bounds only the TCP establishment;
+// without a deadline on the socket a peer that accepts and then goes silent
+// blocks the handshake forever, so the context deadline is applied to conn
+// for the duration of the handshake. conn is closed on any error.
+func Handshake(ctx context.Context, conn net.Conn, addr string, config *ssh.ClientConfig) (*ssh.Client, error) {
+	if deadline, ok := ctx.Deadline(); ok {
+		if err := conn.SetDeadline(deadline); err != nil {
+			closeHandshake(conn, "conn after deadline error")
+			return nil, fmt.Errorf("set handshake deadline: %w", err)
+		}
+	}
+
+	sshConn, chans, reqs, err := ssh.NewClientConn(conn, addr, config)
+	if err != nil {
+		closeHandshake(conn, "conn after handshake error")
+		return nil, fmt.Errorf("ssh handshake: %w", err)
+	}
+
+	if err := conn.SetDeadline(time.Time{}); err != nil {
+		closeHandshake(sshConn, "ssh conn after deadline clear error")
+		return nil, fmt.Errorf("clear handshake deadline: %w", err)
+	}
+
+	return ssh.NewClient(sshConn, chans, reqs), nil
+}
+
+func closeHandshake(c io.Closer, label string) {
+	if err := c.Close(); err != nil {
+		log.Debugf("ssh: close %s: %v", label, err)
+	}
+}