Don't include files using a relative path; always run through the include_path

Don't include files using a relative path; always run through the include_path
explicitly trying absolute paths.

This avoids PHP checking the local directory for a file even if '.' isn't in the
include_path. Also, we now use realpath() directly instead of
Horde_Util::realPath(). This both reduces dependencies, and it removes invalid
paths from the include_path. The generic realPath() has its use, but here we're
dealing with real files on the filesystem.