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/circle.yml

@@ -0,0 +1,26 @@
+version: 2
+jobs:
+  build-go-latest:
+    docker:
+    - image: golang:latest
+    working_directory: /go/src/github.com/gliderlabs/ssh
+    steps:
+    - checkout
+    - run: go get
+    - run: go test -v -race
+
+  build-go-1.9:
+    docker:
+    - image: golang:1.9
+    working_directory: /go/src/github.com/gliderlabs/ssh
+    steps:
+    - checkout
+    - run: go get
+    - run: go test -v -race
+
+workflows:
+  version: 2
+  build:
+    jobs:
+      - build-go-latest
+      - build-go-1.9