Make Refuses to open symlinks msg less spurious

If we're racing enough we could complain about symlinks where there were
none.  This was very surprising.  Make it surprise less.

We should really #ifndef O_NOFOLLOW that code chunk too, for the obvious
reason that we don't need to worry about symlinks if we have and use
O_NOFOLLOW.

Also, since all uses of fcc_open() use O_NOFOLLOW we should move that
into fcc_open().  Ditto O_BINARY and O_CLOEXEC.
Fix racy file ccache corruption in cred_delete()

We *really* need a ccache instance ID tag.  In fact, we should probably
never krb5_cc_initialize() a ccache that doesn't have such a tag.  But
for now cred_delete() is now safe enough.