version: use OSS repo's version when building.

When building with redo, also include the git commit hash
from the proprietary repo, so that we have a precise commit
that identifies all build info (including Go toolchain version).

Add a top-level build script demonstrating to downstream distros
how to burn the right information into builds.

Adjust `tailscale version` to print commit hashes when available.

Fixes #841.

Signed-off-by: David Anderson <danderson@tailscale.com>

logpolicy/logpolicy.go

@@ -395,7 +395,7 @@ func New(collection string) *Policy {
 	log.SetOutput(lw)
 
 	log.Printf("Program starting: v%v, Go %v: %#v",
-		version.LONG,
+		version.Long,
 		strings.TrimPrefix(runtime.Version(), "go"),
 		os.Args)
 	log.Printf("LogID: %v", newc.PublicID)