ipn/ipnauth: don't crash on OpenBSD trying to log username of unknown peer

We never implemented the peercred package on OpenBSD (and I just tried
again and failed), but we've always documented that the creds pointer
can be nil for operating systems where we can't map the unix socket
back to its UID. On those platforms, we set the default unix socket
permissions such that only the admin can open it anyway and we don't
have a read-only vs read-write distinction. OpenBSD was always in that
camp, where any access to Tailscale's unix socket meant full access.

But during some refactoring, we broke OpenBSD in that we started
assuming during one logging path (during login) that Creds was non-nil
when looking up an ipnauth.Actor's username, which wasn't relevant (it
was called from a function "maybeUsernameOf" anyway, which threw away
errors).

Verified on an OpenBSD VM. We don't have any OpenBSD integration tests yet.

Fixes #17209
Updates #17221

(cherry picked from commit 8ec07b5f7f, without
 the one semantic change, limiting it to just the safest part of the fix)

Change-Id: I473c5903dfaa645694bcc75e7f5d484f3dd6044d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnauth/ipnauth.go

@@ -64,7 +64,7 @@ type ConnIdentity struct {
 
 	// Fields used when NotWindows:
 	isUnixSock bool            // Conn is a *net.UnixConn
-	creds      *peercred.Creds // or nil
+	creds      *peercred.Creds // or nil if peercred.Get was not implemented on this OS
 
 	// Used on Windows:
 	// TODO(bradfitz): merge these into the peercreds package and

ipn/ipnserver/actor.go

@@ -145,7 +145,11 @@ func (a *actor) Username() (string, error) {
 		defer tok.Close()
 		return tok.Username()
 	case "darwin", "linux", "illumos", "solaris", "openbsd":
-		uid, ok := a.ci.Creds().UserID()
+		creds := a.ci.Creds()
+		if creds == nil {
+			return "", errors.New("peer credentials not implemented on this OS")
+		}
+		uid, ok := creds.UserID()
 		if !ok {
 			return "", errors.New("missing user ID")
 		}