Work around an strtoul() misfeature.

Work around an strtoul() misfeature.

* gettext-runtime/src/ngettext.c: Include <ctype.h>.
(main): When parsing a nonnegative number, disallow leading whitespace and sign.
* gettext-tools/tests/tstngettext.c: Include <ctype.h>.
(worker_thread): When parsing a nonnegative number, disallow leading whitespace
and sign.
* gnulib-local/lib/markup.c (unescape_string_inplace): When parsing a numeric
character entity, disallow leading whitespace and sign, and report a too large
number as "non-permitted character" rather than "Numerical result out of range".
* gettext-tools/src/msgfmt.c (main): When parsing the '-a' argument, disallow
leading whitespace and sign and check whether the value is a power of 2.
* gettext-tools/src/read-stringtable.c: Include c-ctype.h.
(comment_line_end): When parsing a line number, disallow leading whitespace and
sign.