Extend `line-argument` to return argv with no args

[wasamasa]

This increases Openlisp compatibility once again.

The only incompatibility I see is that (line-argument 0) returns the command executed on Openlisp, but returns the first argument on eisl. (line-argument 1) returns the first argument on Openlisp, but returns the second argument on eisl. This makes writing interoperable programs difficult.

[sasagawa888]

Thank you.