bluetooth: add soft rfkill unblock, wait for adapter to become available

instead of forfeiting subscription

fixes #2922
fixes #1537

(cherry picked from commit 77a357109a)

core/internal/server/bluez/rfkill_linux.go

@@ -0,0 +1,19 @@
+package bluez
+
+import "os"
+
+// linux/rfkill.h: struct rfkill_event { __u32 idx; __u8 type; __u8 op; __u8 soft; __u8 hard; },
+// RFKILL_TYPE_BLUETOOTH=2, RFKILL_OP_CHANGE_ALL=3
+func rfkillUnblockBluetooth() error {
+	f, err := os.OpenFile("/dev/rfkill", os.O_WRONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	var event [8]byte
+	event[4] = 2
+	event[5] = 3
+	_, err = f.Write(event[:])
+	return err
+}