Expose socket interruption

On Linux, socket functions are unblocked by shutdown(), but on Windows
they are unblocked by closesocket().

Expose net_interrupt() and net_close() to abstract these differences:
 - net_interrupt() calls shutdown() on Linux and closesocket() on
   Windows (if not already called);
 - net_close() calls close() on Linux and closesocket() on Windows (if
   not already called).

This simplifies the server code, and prevents a data race on close
(reported by TSAN) on Linux (but does not fix it on Windows):

    WARNING: ThreadSanitizer: data race (pid=836124)
      Write of size 8 at 0x7ba0000000d0 by main thread:
        #0 close ../../../../src/libsanitizer/tsan/tsan_interceptors_posix.cpp:1690 (libtsan.so.0+0x359d8)
        #1 net_close ../app/src/util/net.c:211 (scrcpy+0x1c76b)
        #2 close_socket ../app/src/server.c:330 (scrcpy+0x19442)
        #3 server_stop ../app/src/server.c:522 (scrcpy+0x19e33)
        #4 scrcpy ../app/src/scrcpy.c:532 (scrcpy+0x156fc)
        #5 main ../app/src/main.c:92 (scrcpy+0x622a)

      Previous read of size 8 at 0x7ba0000000d0 by thread T6:
        #0 recv ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:6603 (libtsan.so.0+0x4f4a6)
        #1 net_recv ../app/src/util/net.c:167 (scrcpy+0x1c5a7)
        #2 run_receiver ../app/src/receiver.c:76 (scrcpy+0x12819)
        #3 <null> <null> (libSDL2-2.0.so.0+0x84f40)

app/src/util/net.c

@@ -1,5 +1,6 @@
 #include "net.h"
 
+#include <assert.h>
 #include <stdio.h>
 #include <SDL2/SDL_platform.h>
 
@@ -55,6 +56,7 @@ wrap(sc_raw_socket sock) {
     }
 
     socket->socket = sock;
+    socket->closed = (atomic_flag) ATOMIC_FLAG_INIT;
 
     return socket;
 #else
@@ -195,18 +197,33 @@ net_send_all(sc_socket socket, const void *buf, size_t len) {
 }
 
 bool
-net_shutdown(sc_socket socket, int how) {
+net_interrupt(sc_socket socket) {
+    assert(socket != SC_INVALID_SOCKET);
+
     sc_raw_socket raw_sock = unwrap(socket);
-    return !shutdown(raw_sock, how);
+
+#ifdef __WINDOWS__
+    if (!atomic_flag_test_and_set(&socket->closed)) {
+        return !closesocket(raw_sock);
+    }
+    return true;
+#else
+    return !shutdown(raw_sock, SHUT_RDWR);
+#endif
 }
 
+#include <errno.h>
 bool
 net_close(sc_socket socket) {
     sc_raw_socket raw_sock = unwrap(socket);
 
 #ifdef __WINDOWS__
+    bool ret = true;
+    if (!atomic_flag_test_and_set(&socket->closed)) {
+        ret = !closesocket(raw_sock);
+    }
     free(socket);
-    return !closesocket(raw_sock);
+    return ret;
 #else
     return !close(raw_sock);
 #endif