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/wrap.go

@@ -0,0 +1,33 @@
+package ssh
+
+import gossh "golang.org/x/crypto/ssh"
+
+// PublicKey is an abstraction of different types of public keys.
+type PublicKey interface {
+	gossh.PublicKey
+}
+
+// The Permissions type holds fine-grained permissions that are specific to a
+// user or a specific authentication method for a user. Permissions, except for
+// "source-address", must be enforced in the server application layer, after
+// successful authentication.
+type Permissions struct {
+	*gossh.Permissions
+}
+
+// A Signer can create signatures that verify against a public key.
+type Signer interface {
+	gossh.Signer
+}
+
+// ParseAuthorizedKey parses a public key from an authorized_keys file used in
+// OpenSSH according to the sshd(8) manual page.
+func ParseAuthorizedKey(in []byte) (out PublicKey, comment string, options []string, rest []byte, err error) {
+	return gossh.ParseAuthorizedKey(in)
+}
+
+// ParsePublicKey parses an SSH public key formatted for use in
+// the SSH wire protocol according to RFC 4253, section 6.6.
+func ParsePublicKey(in []byte) (out PublicKey, err error) {
+	return gossh.ParsePublicKey(in)
+}