Fix some #include problems.

- Include files by relative paths for installed headers.
- Install all headers used by other installed headers.
- Use
    #include "..."
  for our own headers and
    #include <...>
  for system headers.
- Remove one unused header.

Updated the includedeps.py/.cmake script to be able to check also legacy
stuff without complaining too much.  As part of this, updated the
installation rules for legacyheaders to explicitly specify the list of
files (using globbing).

The script now also includes some new directories into the include
dependency graph generated by 'make depgraphs'.  Will try to make the
graph a bit more useful at some point.

Change-Id: Id24d37672194770ebd7552b7d5c7d8bc964c0ffb