ed: better relative addresses

[mknos]

• The initial implementation of relative addresses was too limited
• GNU and OpenBSD ed allow repeated +n, mixed +n and --++, as well as basing address from dot, dollar or a marked address

For tests, current address was set to 10.

3+3=    6 (+n pattern applied to literal 3)
3++=    5 (++ applied to literal)
++1=    invalid: ambiguous
+=      current + 1
.+=     current + 1
+1=     current + 1
.+1=    current + 1
.+3++=  current + 5: ++ follows +n
$---=   end - 3
$-3=    end - 3
$-3+3=  end (repeated +n)
'x-5,'x+5nl      list 5 lines of context around marked line x

```

[briandfoy]

changes: allow repeated - and + for relative addresses, such as 3++ or $--10