ipn{,/local},cmd/tailscale: add "sync" flag and pref to disable control map poll

For manual (human) testing, this lets the user disable control plane
map polls with "tailscale set --sync=false" (which survives restarts)
and "tailscale set --sync" to restore.

A high severity health warning is shown while this is active.

Updates #12639
Updates #17945

Change-Id: I83668fa5de3b5e5e25444df0815ec2a859153a6d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

types/opt/bool.go

@@ -83,6 +83,17 @@ func (b *Bool) Scan(src any) error {
 	}
 }
 
+// Normalized returns the normalized form of b, mapping "unset" to ""
+// and leaving other values unchanged.
+func (b Bool) Normalized() Bool {
+	switch b {
+	case ExplicitlyUnset:
+		return Empty
+	default:
+		return b
+	}
+}
+
 // EqualBool reports whether b is equal to v.
 // If b is empty or not a valid bool, it reports false.
 func (b Bool) EqualBool(v bool) bool {