A file name as a string, including its directory path, as defined by the operating system.
This function reads the filename and directory path as a string, returning the directory path as a string.
Gamma> x = dirname("/usr/george/lib/misc/myfile.dat");
"/usr/george/lib/misc"
Gamma> y = dirname("misc/myfile.dat");
"misc"
Gamma> z = dirname("myfile.dat");
"myfile.dat"
Gamma>