Fix System.Process when calling with absolute path and directory change
Currently, calling with an absolute path to a binary (e.g.
/opt/clean/bin/clm) and a directory change (e.g.
newdir) yields an error (
newdir//opt/clean/bin/clm does not exist). This removes the check, which is easier than checking if it concerns an absolute path. It is also not clear why file existence should be checked before the fork, but, for instance, file permissions should not.