switch to use gliderlabs/ssh for builtin server (#7250)

resolves git conflicts from #3896 (credit to @belak, in case github doesn't keep original author during squash)

Co-Authored-By: Matti Ranta <techknowlogick@gitea.io>

vendor/github.com/gliderlabs/ssh/conn.go

@@ -0,0 +1,55 @@
+package ssh
+
+import (
+	"context"
+	"net"
+	"time"
+)
+
+type serverConn struct {
+	net.Conn
+
+	idleTimeout   time.Duration
+	maxDeadline   time.Time
+	closeCanceler context.CancelFunc
+}
+
+func (c *serverConn) Write(p []byte) (n int, err error) {
+	c.updateDeadline()
+	n, err = c.Conn.Write(p)
+	if _, isNetErr := err.(net.Error); isNetErr && c.closeCanceler != nil {
+		c.closeCanceler()
+	}
+	return
+}
+
+func (c *serverConn) Read(b []byte) (n int, err error) {
+	c.updateDeadline()
+	n, err = c.Conn.Read(b)
+	if _, isNetErr := err.(net.Error); isNetErr && c.closeCanceler != nil {
+		c.closeCanceler()
+	}
+	return
+}
+
+func (c *serverConn) Close() (err error) {
+	err = c.Conn.Close()
+	if c.closeCanceler != nil {
+		c.closeCanceler()
+	}
+	return
+}
+
+func (c *serverConn) updateDeadline() {
+	switch {
+	case c.idleTimeout > 0:
+		idleDeadline := time.Now().Add(c.idleTimeout)
+		if idleDeadline.Unix() < c.maxDeadline.Unix() || c.maxDeadline.IsZero() {
+			c.Conn.SetDeadline(idleDeadline)
+			return
+		}
+		fallthrough
+	default:
+		c.Conn.SetDeadline(c.maxDeadline)
+	}
+}