This efun is available only if __OLD_ED__ is not defined.
The internal editor is started, optionally loading 'file' for editing.
The resulting output is returned. The editor session remains active,
and further calls to ed_cmd() can be used to send commands to it.
If restricted is 1, the commands that change which file is being edited
will be disabled.
If scroll_lines is nonzero, it sets the number of lines displayed by the
editor's scrolling commands (such as 'z'). The default is 20.
When ed_start() is called with exactly two arguments, a second argument
equal to 1 is treated as 'restricted'; any other value is treated as
'scroll_lines'.
Only one ed session can be active per object at a time.