Add check_cloexec_fds debug command

Add check_cloexec_fds debug command

Add a debug command (to be executed manually via Alt+F2) to check
that all of gnome-shell's file descriptors have the CLOEXEC flag set.
This is important so that internal file descriptors do not get passed
to apps when they are launched.

It prints a warning message for every fd that does not have the flag set.

fdwalk() is used from the standard library if available (it is not
available in glibc), otherwise we use the same implementation as glib
has internally.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/132