Undo an unadvertised change in the API of pg_atoi. In all previous

Undo an unadvertised change in the API of pg_atoi.  In all previous
releases, a nonzero 'c' argument meant that the input string could be
terminated by either that character or \0.  Recent refactoring broke
that, causing the thing to scan for 'c' only.  This went undetected
because no part of the main code actually passes nonzero 'c'.  However
it broke tsearch2 and possibly other user-written code that assumed
the old definition.  Per report from Tom Hebbron.