This adds cross-platform support for it: on glibc, musl and BSD’s libc,
the flag is natively supported. On Windows, convert it to the `N` flag,
which similarly indicates that an open file shouldn’t be inherited by
child processes.

This allows us to unconditionally pass `e` to `g_fopen()` so `O_CLOEXEC`
can easily be set on its FDs.

Also do the same for `g_freopen()`, since it shares the same underlying
mode handling code.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>