If the locale's positive_sign and negative_sign values would both be
returned by localeconv() as empty strings, strfmon() shall behave as if
the negative_sign value was the string "-".

This occurs with the C locale. The implementation now checks that
int_{n,p}_sign_posn and {n,p}_sign_posn are properly initialized, as per
localeconv(3) - valid values are 0-4. If not initialized, they are set
to 1, which positions the sign before the string.

Austin Group Defect 1199 is applied, changing the requirements for the
'+' and '(' flags.