isnanl returns true for 2 and 3 on x86 openbsd 3.9

[Jim Meyering]

Hi Bruno,

  openbsd$ ./seq 4
  1
  nan
  nan
  4

I tracked this to printf-posix's use of isnanl.
That function from gnulib returns true for the long-double
values of 2 and 3 (exponent is 0 in those cases, yet this
code returns 1:

  if (exponent == 0)
    return (m.word[1] >> 31);

I don't have time to investigate further now.
Maybe tomorrow.