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/anmitsu/go-shlex/README.md

@@ -0,0 +1,38 @@
+# go-shlex
+
+go-shlex is a library to make a lexical analyzer like Unix shell for
+Go.
+
+## Install
+
+    go get -u "github.com/anmitsu/go-shlex"
+
+## Usage
+
+```go
+package main
+
+import (
+    "fmt"
+    "log"
+
+    "github.com/anmitsu/go-shlex"
+)
+
+func main() {
+    cmd := `cp -Rdp "file name" 'file name2' dir\ name`
+    words, err := shlex.Split(cmd, true)
+    if err != nil {
+        log.Fatal(err)
+    }
+
+    for _, w := range words {
+        fmt.Println(w)
+    }
+}
+```
+
+## Documentation
+
+http://godoc.org/github.com/anmitsu/go-shlex
+