Patch: Handle relative paths in .debug_line

[Jim Blandy]

Thanks for this patch!

In the revised dwarf2_start_subfile:

- It looks to me as if the new code doesn't handle the case where both
  dirname and comp_dir are null.

- It seems to me that, when fullname == filename, the loop over the
  subfiles duplicates the loop at the top of start_subfile.  So that
  loop should remain conditional.  Its comment should be moved and
  adjusted, too.