input: avoid creating world-writeable file with --input-unix-socket

This requires fchmod(), which is not necessarily available everywhere.
It also might not work at all. (It does work on Linux.)

input/ipc.c

@@ -725,6 +725,10 @@ static void *ipc_thread(void *p)
         goto done;
     }
 
+#if HAVE_FCHMOD
+    fchmod(ipc_fd, 0600);
+#endif
+
     size_t path_len = strlen(arg->path);
     if (path_len >= sizeof(ipc_un.sun_path) - 1) {
         MP_ERR(arg, "Could not create IPC socket\n");

old-configure

@@ -972,6 +972,7 @@ cat > $TMPC << EOF
 #define HAVE_BSD_THREAD_NAME 0
 #define HAVE_NETBSD_THREAD_NAME 0
 #define HAVE_DXVA2_HWACCEL 0
+#define HAVE_FCHMOD 0
 
 #define DEFAULT_CDROM_DEVICE "/dev/cdrom"
 #define DEFAULT_DVD_DEVICE   "/dev/dvd"

wscript

@@ -218,6 +218,10 @@ iconv support use --disable-iconv.",
         'deps_neg': [ 'glob' ],
         'deps_any': [ 'os-win32', 'os-cygwin' ],
         'func': check_true
+    }, {
+        'name': 'fchmod',
+        'desc': 'fchmod()',
+        'func': check_statement('sys/stat.h', 'fchmod(0, 0)'),
     }, {
         'name': 'glibc-thread-name',
         'desc': 'GLIBC API for setting thread name',