#400 `set dashtype cycle` option

[Andrew Rasmussen]

To emulate the old default behavior of cycling through terminal-specific dash types, it would be useful to have a command like set dashtype cycle, similar to set linetype cycle. This is a very helpful feature, especially for the color blind and those using monochrome terminals (or publishing in black-and-white).

At present 5.0 requires that users manually change the dash type for each new line plotted (unless I am missing something).