More strtod touchups. * tests/test-strtod.c (main): Ignore tests for signbit on NaN, and sign of negative underflow, for now. Use .5, not .1. * doc/posix-functions/strtod.texi (strtod): Mention these limitations. Reported by Jim Meyering. Signed-off-by: Eric Blake <ebb9@byu.net>