rrdtool: Improve argument list modification macro

rrdtool: Improve argument list modification macro

The name of the macro was not very descriptive, "add_arg" describes
the functionality much better.

Instead of modifying some magic function variables, give the variables
as parameters to the macro. This makes it much more clear how the
thing actually works.

Signed-off-by: Timo Kokkonen <kaapeli@itanic.dy.fi>