Document various strtod bugs, with some fixes.

Document various strtod bugs, with some fixes.

* doc/posix-functions/strtod.texi (strtod): Document bugs with
"-0x", "inf", "nan", and hex constants.
* doc/posix-functions/atof.texi (atof): Likewise.
* modules/stdlib (Makefile.am): Support strtod.
* m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Likewise.
* m4/strtod.m4 (gl_FUNC_STRTOD): Fit in stdlib framework, and
detect additional strtod bugs.
* lib/stdlib.in.h (rpl_strtod): Add declarations.
* lib/strtod.c (strtod): Return -0.0 on negative underflow.  Use
bool where appropriate.  Parse 'inf' and 'nan'.
* tests/test-strtod.c: New file.
* modules/strtod (Depends-on): Add stdbool, stdlib.
(configure.ac): Turn on module indicator.
* modules/strtod-tests: New module.

Signed-off-by: Eric Blake <ebb9@byu.net>