Behaviour of fflush() on a file opened for reading

[Colin Percival]

  PR kern/65402 complains about fflush() failing if called
on a file opened read-only.  Our libc returns an error; many
do not.  According to the ISO C standard, behaviour here is
undefined.
  Should we change our behaviour to meet the expectations of
broken programs?  In order to avoid creating lots of noise
on the list, please reply to me directly.

Colin Percival