A little spot of code for an item from the todo list,

        * srfi-1.c, srfi-1.h, srfi-1.scm (list-copy): New function, derived
        from core list-copy but allowing improper lists, per SRFI-1 spec.

        * tests/srfi-1.test (list-copy): New tests.

I think this should go in the 1.6 branch too.

I suppose it'd be easier to relax the core list-copy, but for now at
least a version in the srfi-1 module can do what srfi-1 says it should.