Restored removal of the special handling needed for a result of +-0.

It was lost in rev.1.9.  The log message for rev.1.9 says that the
special case of +-0 is handled twice, but it was only handled once,
so it became unhandled, and this happened to break half of the cases
that return +-0:
- round-towards-minus-infinity:  0   <  x < 1:  result was -0 not  0
- round-to-nearest:             -0.5 <= x < 0:  result was  0 not -0
- round-towards-plus-infinity:  -1   <  x < 0:  result was  0 not -0
- round-towards-zero:           -1   <  x < 0:  result was  0 not -0

lib/msun/src/s_rintf.c

@@ -40,6 +40,8 @@ rintf(float x)
 		if((i0&0x7fffffff)==0) return x;
 	        w = TWO23[sx]+x;
 	        t =  w-TWO23[sx];
+		GET_FLOAT_WORD(i0,t);
+		SET_FLOAT_WORD(t,(i0&0x7fffffff)|(sx<<31));
 	        return t;
 	    }
 	    w = TWO23[sx]+x;