Line numbers start at 1

Line numbers start at 1

Bumps the line number for the singleton warning. When the singleton
occurs on the same line at the term starts, the line number is correct:

    foo(X).

However it stills mis-reports this line number as 1 instead of 5:

    foo(1) :-
        true,
        true,
        true,
        Y.

See issue #1356.