Remove more obsolete content from share/html.

- yourown.html was not referenced.
- Will replace ADD_INCLUDE and include_*.html with something more useful
  in the future.
- FAQ link in online.html was referencing a file deleted in release-4-6.
- Other changes remove content that was referencing share/tutor/, which
  was removed earlier.

Related to #1242.

Change-Id: I0e04438694cf251eae789c08e2ae1087d6fd99fc
Move binary directory search to ProgramInfo.

This makes it possible to print the real executable location in the
startup header.  Probably useful diagnostic if one has multiple Gromacs
installations in different paths.  Also, now the search for the path is
done at most once per invocation, and not for each library file.

This is mainly moving code around and converting it to C++:
- Part of get_libdir() gets moved into gmx::ProgramInfo, and the
  remainder is now using C++.
- Move gmx_is_file() to gmx::File, since it is only used in the code
  moved to gmx::ProgramInfo.

There are some minor behavior changes:
- The search now works correctly if PATH contains an empty entry
  (i.e., the working directory is in the path).
- If PATH does not contain the working directory explicity, the search
  only considers the working directory on Windows.
- If the working directory has been changed in code (but not before call
  to gmx::init()), the search still works.

Change-Id: If1d548be6b4dd031617a6019bcad20466ad91994