fd-util: Move O_ACCMODE_STRICT to fd-util.h

fd-util: Move O_ACCMODE_STRICT to fd-util.h

We're still discussing whether we want to remove this or not, but
regardless of whether we end up removing it or not, it's something
we add ourselves and as such should not live in our headers that
override glibc headers, so let's move it to fd-util.h.