afprog: Fix the grammar so that program() source works again

The grammar of the afprog module was a bit flaky, which resulted in
program() sources not being recognised, because the grammar couldn't
properly terminate the sentence, and skipped it instead. By refactoring
the grammar into the same style the program() destination uses, the
problem went away. This closes #34.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>