Miscellaneous fix, make `org-goto-first-child' behave intuitively before first heading

[Gustav Wikström]

Hi again,

While fixing other issues in the code I found thing worth improving. Thought to 
just briefly mention it here as well, for full transparency. 
`org-goto-first-child' now understand what a child is when before first 
heading. This is inline with how other outline functions behave.

Since this change, and the other changes I've made, are relatively small I 
haven't taken them through any review process on the list. To save us all of 
some bureaucracy.
 
--- Commit dad436c60
Make `org-goto-first-child' behave intuitively before first heading

* lisp/org.el (org-goto-first-child): Make it understand outline
level 0 as well.  The function now behaves intuitively also before
first heading.

Regards
Gustav