Clean up xvgr.h and viewit.h usage

- Remove unnecessary includes of these files.
- Include viewit.h in files that were covered transitively through
  xvgr.h, and remove the include from there.
- Remove other unnecessary includes in these headers.
- Make can_view() static in viewit.c, since it wasn't even declared in
  the header.

Change-Id: I7b6b3724a5b7fd40be46eb8fbcb9d9aeaccf9cc8