Store actual serial in sc_server

Before starting the server, the actual device serial (possibly its
ip:port if it's over TCP/IP) must be known.

A serial might be requested via -s/--serial (stored in the
sc_server_params), but the actual serial may change afterwards:
 - if none is provided, then it is retrieved with "adb get-serialno";
 - if --tcpip is requested, then the final serial will be the target
   ip:port.

The requested serial was overwritten by the actual serial in the
sc_server_params struct, which was a bit hacky.

Instead, store a separate serial field in sc_server (and rename the one
from sc_server_params to "req_serial" to avoid confusion).

app/src/server.h

@@ -22,7 +22,7 @@ struct sc_server_info {
 };
 
 struct sc_server_params {
-    const char *serial;
+    const char *req_serial;
     enum sc_log_level log_level;
     const char *crop;
     const char *codec_options;
@@ -49,6 +49,7 @@ struct sc_server_params {
 struct sc_server {
     // The internal allocated strings are copies owned by the server
     struct sc_server_params params;
+    char *serial;
 
     sc_thread thread;
     struct sc_server_info info; // initialized once connected