Extract adb tunnel to a separate component

This simplifies the server code.

app/src/adb_tunnel.h

@@ -0,0 +1,47 @@
+#ifndef SC_ADB_TUNNEL_H
+#define SC_ADB_TUNNEL_H
+
+#include "common.h"
+
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "options.h"
+#include "util/intr.h"
+#include "util/net.h"
+
+struct sc_adb_tunnel {
+    bool enabled;
+    bool forward; // use "adb forward" instead of "adb reverse"
+    sc_socket server_socket; // only used if !forward
+    uint16_t local_port;
+};
+
+/**
+ * Initialize the adb tunnel struct to default values
+ */
+void
+sc_adb_tunnel_init(struct sc_adb_tunnel *tunnel);
+
+/**
+ * Open a tunnel
+ *
+ * Blocking calls may be interrupted asynchronously via `intr`.
+ *
+ * If `force_adb_forward` is not set, then attempts to set up an "adb reverse"
+ * tunnel first. Only if it fails (typical on old Android version connected via
+ * TCP/IP), use "adb forward".
+ */
+bool
+sc_adb_tunnel_open(struct sc_adb_tunnel *tunnel, struct sc_intr *intr,
+                   const char *serial, struct sc_port_range port_range,
+                   bool force_adb_forward);
+
+/**
+ * Close the tunnel
+ */
+bool
+sc_adb_tunnel_close(struct sc_adb_tunnel *tunnel, struct sc_intr *intr,
+                    const char *serial);
+
+#endif