Fill in the 'Section' field in debian/control correctly, or not at all

[euanh]

The 'Section' field in the debian/control file is only 'recommended', not 'required': https://www.debian.org/doc/debian-policy/ch-controlfields.html#s-sourcecontrolfiles. In any case, makedeb.py currently sets this field to 'ocaml'. We should either remove this field or fill it correctly.