Cleanup version introspection

* Unify version checking code into version.go
* Make 'version' available as a job in the engine
* Use simplified version checking code when setting user agent for registry client.

version.go

@@ -0,0 +1,33 @@
+package docker
+
+import (
+	"github.com/dotcloud/docker/engine"
+	"github.com/dotcloud/docker/utils"
+	"runtime"
+)
+
+func init() {
+	engine.Register("version", jobVersion)
+}
+
+func jobVersion(job *engine.Job) engine.Status {
+	if _, err := dockerVersion().WriteTo(job.Stdout); err != nil {
+		job.Errorf("%s", err)
+		return engine.StatusErr
+	}
+	return engine.StatusOK
+}
+
+// dockerVersion returns detailed version information in the form of a queriable
+// environment.
+func dockerVersion() *engine.Env {
+	v := &engine.Env{}
+	v.Set("Version", VERSION)
+	v.Set("GitCommit", GITCOMMIT)
+	v.Set("GoVersion", runtime.Version())
+	// FIXME:utils.GetKernelVersion should only be needed here
+	if kernelVersion, err := utils.GetKernelVersion(); err == nil {
+		v.Set("KernelVersion", kernelVersion.String())
+	}
+	return v
+}