Various fixes related to `org-insert-heading'.

Various fixes related to `org-insert-heading'.

* org.el (org-previous-line-empty-p): New parameter to allow
checking next line.  Add a docstring.
(org-insert-heading): Handle two universal prefix arguments as
advertized in the docstring.  Don't insert new lines when
creating a heading after the first heading in the current
subtree.
(org-insert-heading-respect-content): New optional argument
arg, passed to `org-insert-heading'.

* org.texi (Structure editing): Update documentation for
`org-insert-heading-or-item'.
(Plain lists, Relative timer): Update index entry.

* org-mobile.el (org-mobile-edit): Use correct parameters for
`org-insert-heading-respect-content'.