LibCore: Open files with O_CLOEXEC by default

This changes Core::File::open() to specify O_CLOEXEC by default
so that we don't leak file descriptors into child processes. The
new behavior can be overriden by specifying OpenMode::KeepOnExec.

Userland/Libraries/LibCore/IODevice.h

@@ -42,6 +42,7 @@ enum class OpenMode : unsigned {
     Append = 4,
     Truncate = 8,
     MustBeNew = 16,
+    KeepOnExec = 32,
 };
 
 enum class SeekMode {