* stdlib/stdlib.h: Remove warn_unused_result attribute from strtol etc.

[BZ #13970]
(strtod, strtof, strtold, strtol, strtoul, strtoq)
(strtouq, strtoll, strtoull, strtol_l, strtoul_l, strtoll_l, strtoull_l)
(strtod_l, strtof_l, strtold_l): Remove __wur.
It is not necessarily an error to ignore strtol's return value.
One can reliably look at the stored endptr to decide whether
the number had valid syntax.