Enable addition of top-level functions in autodoc.

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1900

Reported by Martin Nilsson, IDA nilsson@pike.ida.liu.se

It is currently not possible to parse the master.pike.in for autodoc. The main reason is that is must be possible to add functions on top level. These top level functions is currently marked with the unknown keyword @global.