build, cmd/stdiscosrv, cmd/strelaysrv: Rename binaries to add "st" prefix

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3371

cmd/stdiscosrv/README.md

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+stdiscosrv
+==========
+
+[![Latest Build](http://img.shields.io/jenkins/s/http/build.syncthing.net/stdiscosrv.svg?style=flat-square)](http://build.syncthing.net/job/stdiscosrv/lastBuild/)
+
+This is the global discovery server for the `syncthing` project.
+
+To get it, run `go get github.com/syncthing/stdiscosrv` or download the
+[latest build](http://build.syncthing.net/job/stdiscosrv/lastSuccessfulBuild/artifact/)
+from the build server.
+
+Usage
+-----
+
+The discovery server supports `ql` and `postgres` backends.
+Specify the backend via `-db-backend` and the database DSN via `-db-dsn`.
+
+By default it will use in-memory `ql` backend. If you wish to persist the
+information on disk between restarts in `ql`, specify a file DSN:
+
+```bash
+$ stdiscosrv -db-dsn="file:///var/run/stdiscosrv.db"
+```
+
+For `postgres`, you will need to create a database and a user with permissions
+to create tables in it, then start the stdiscosrv as follows:
+
+```bash
+$ export STDISCOSRV_DB_DSN="postgres://user:password@localhost/databasename"
+$ stdiscosrv -db-backend="postgres"
+```
+
+You can pass the DSN as command line option, but the value what you pass in will
+be visible in most process managers, potentially exposing the database password
+to other users.
+
+In all cases, the appropriate tables and indexes will be created at first
+startup. If it doesn't exit with an error, you're fine.
+
+See `stdiscosrv -help` for other options.