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/util.go

@@ -0,0 +1,83 @@
+package ssh
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"encoding/binary"
+
+	"golang.org/x/crypto/ssh"
+)
+
+func generateSigner() (ssh.Signer, error) {
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		return nil, err
+	}
+	return ssh.NewSignerFromKey(key)
+}
+
+func parsePtyRequest(s []byte) (pty Pty, ok bool) {
+	term, s, ok := parseString(s)
+	if !ok {
+		return
+	}
+	width32, s, ok := parseUint32(s)
+	if !ok {
+		return
+	}
+	height32, _, ok := parseUint32(s)
+	if !ok {
+		return
+	}
+	pty = Pty{
+		Term: term,
+		Window: Window{
+			Width:  int(width32),
+			Height: int(height32),
+		},
+	}
+	return
+}
+
+func parseWinchRequest(s []byte) (win Window, ok bool) {
+	width32, s, ok := parseUint32(s)
+	if width32 < 1 {
+		ok = false
+	}
+	if !ok {
+		return
+	}
+	height32, _, ok := parseUint32(s)
+	if height32 < 1 {
+		ok = false
+	}
+	if !ok {
+		return
+	}
+	win = Window{
+		Width:  int(width32),
+		Height: int(height32),
+	}
+	return
+}
+
+func parseString(in []byte) (out string, rest []byte, ok bool) {
+	if len(in) < 4 {
+		return
+	}
+	length := binary.BigEndian.Uint32(in)
+	if uint32(len(in)) < 4+length {
+		return
+	}
+	out = string(in[4 : 4+length])
+	rest = in[4+length:]
+	ok = true
+	return
+}
+
+func parseUint32(in []byte) (uint32, []byte, bool) {
+	if len(in) < 4 {
+		return 0, nil, false
+	}
+	return binary.BigEndian.Uint32(in), in[4:], true
+}