xstrtod, xstrtold: Don't treat gradual underflow as an error.

xstrtod, xstrtold: Don't treat gradual underflow as an error.

* lib/xstrtod.c (XSTRTOD): Upon gradual underflow, return true with
errno = 0.
* lib/xstrtod.h (xstrtod, xstrtold): Update specification.
* tests/test-xstrtod.c (main): For gradual underflow, expect ok &&
errno == 0.
* tests/test-xstrtold.c (main): Likewise.