Implement do_arc

[mojca]

1. define and document a new terminal entry point term->do_arc()
   The definition is in src/term_api.h; documentation is in term/README
2. in the term.c routine change_term(), add a line
   if (term->arc == 0) term->arc = do_arc;
3. wherever the code calls do_arc(), replace this with term->do_arc().
   I think there are only 3 such places.
4. Write your specific routine TERM_do_arc() and initialize its TERM_TABLE to point to it.