c-format: Recognize the %b format directive.
* gettext-tools/src/format-c-parse.h (format_parse_entrails): Recognize %b as
a directive that takes an unsigned integer argument.
* gettext-tools/tests/format-c-1: Add test of %b.
* gettext-tools/tests/format-c-2: Add type compatibility test for %b.
* NEWS: Mention it.
