LibCore: Implement System::set_close_on_exec

Author: https://github.com/stasoid
Commit: 2abc792938
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3002
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/AtkinsSJ

Libraries/LibCore/System.cpp

@@ -1017,4 +1017,17 @@ ErrorOr<void> sleep_ms(u32 milliseconds)
     return {};
 }
 
+ErrorOr<void> set_close_on_exec(int fd, bool enabled)
+{
+    int flags = TRY(fcntl(fd, F_GETFD));
+
+    if (enabled)
+        flags |= FD_CLOEXEC;
+    else
+        flags &= ~FD_CLOEXEC;
+
+    TRY(fcntl(fd, F_SETFD, flags));
+    return {};
+}
+
 }