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/options.go

@@ -0,0 +1,77 @@
+package ssh
+
+import (
+	"io/ioutil"
+
+	gossh "golang.org/x/crypto/ssh"
+)
+
+// PasswordAuth returns a functional option that sets PasswordHandler on the server.
+func PasswordAuth(fn PasswordHandler) Option {
+	return func(srv *Server) error {
+		srv.PasswordHandler = fn
+		return nil
+	}
+}
+
+// PublicKeyAuth returns a functional option that sets PublicKeyHandler on the server.
+func PublicKeyAuth(fn PublicKeyHandler) Option {
+	return func(srv *Server) error {
+		srv.PublicKeyHandler = fn
+		return nil
+	}
+}
+
+// HostKeyFile returns a functional option that adds HostSigners to the server
+// from a PEM file at filepath.
+func HostKeyFile(filepath string) Option {
+	return func(srv *Server) error {
+		pemBytes, err := ioutil.ReadFile(filepath)
+		if err != nil {
+			return err
+		}
+
+		signer, err := gossh.ParsePrivateKey(pemBytes)
+		if err != nil {
+			return err
+		}
+
+		srv.AddHostKey(signer)
+
+		return nil
+	}
+}
+
+// HostKeyPEM returns a functional option that adds HostSigners to the server
+// from a PEM file as bytes.
+func HostKeyPEM(bytes []byte) Option {
+	return func(srv *Server) error {
+		signer, err := gossh.ParsePrivateKey(bytes)
+		if err != nil {
+			return err
+		}
+
+		srv.AddHostKey(signer)
+
+		return nil
+	}
+}
+
+// NoPty returns a functional option that sets PtyCallback to return false,
+// denying PTY requests.
+func NoPty() Option {
+	return func(srv *Server) error {
+		srv.PtyCallback = func(ctx Context, pty Pty) bool {
+			return false
+		}
+		return nil
+	}
+}
+
+// WrapConn returns a functional option that sets ConnCallback on the server.
+func WrapConn(fn ConnCallback) Option {
+	return func(srv *Server) error {
+		srv.ConnCallback = fn
+		return nil
+	}
+}