[ast] limit occurences of argn(0)==0 to improve backward compatibility

[ast] limit occurences of argn(0)==0 to improve backward compatibility

Since https://codereview.scilab.org/#/c/21034/ when a macro
was called without output arguments, the value argn(0)==0 is given
without restriction and this can break a lot of user codes checking
the number of output arguments with constructs like

if argn(0) <> 1, ..., end

instead of

if argn(0) > 1, ..., end

With this patch, argn(0)==0 is only granted when macro prototype has
no formal output argument or if varargout is the only formal output
argument.

Change-Id: Ifc235a2ac986550b41522fddbf448b6eaa02b2d3