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/doc.go

@@ -0,0 +1,45 @@
+/*
+Package ssh wraps the crypto/ssh package with a higher-level API for building
+SSH servers. The goal of the API was to make it as simple as using net/http, so
+the API is very similar.
+
+You should be able to build any SSH server using only this package, which wraps
+relevant types and some functions from crypto/ssh. However, you still need to
+use crypto/ssh for building SSH clients.
+
+ListenAndServe starts an SSH server with a given address, handler, and options. The
+handler is usually nil, which means to use DefaultHandler. Handle sets DefaultHandler:
+
+  ssh.Handle(func(s ssh.Session) {
+      io.WriteString(s, "Hello world\n")
+  })
+
+  log.Fatal(ssh.ListenAndServe(":2222", nil))
+
+If you don't specify a host key, it will generate one every time. This is convenient
+except you'll have to deal with clients being confused that the host key is different.
+It's a better idea to generate or point to an existing key on your system:
+
+  log.Fatal(ssh.ListenAndServe(":2222", nil, ssh.HostKeyFile("/Users/progrium/.ssh/id_rsa")))
+
+Although all options have functional option helpers, another way to control the
+server's behavior is by creating a custom Server:
+
+  s := &ssh.Server{
+      Addr:             ":2222",
+      Handler:          sessionHandler,
+      PublicKeyHandler: authHandler,
+  }
+  s.AddHostKey(hostKeySigner)
+
+  log.Fatal(s.ListenAndServe())
+
+This package automatically handles basic SSH requests like setting environment
+variables, requesting PTY, and changing window size. These requests are
+processed, responded to, and any relevant state is updated. This state is then
+exposed to you via the Session interface.
+
+The one big feature missing from the Session abstraction is signals. This was
+started, but not completed. Pull Requests welcome!
+*/
+package ssh