rrdtool: Set up the correct executable name when running processes

rrdtool: Set up the correct executable name when running processes

The first argument, the name of the executable, should be set properly
prior running execv(). This ensures the process knows properly how it
was called. There might be a difference in case the process implements
multiple personalities based on the name it was called.

This does not appear to be the case with rrdtool as it was working
even if the executable name was left blank.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>