Improvements in DH_OPTIONS handling and DH_AUTO_OPTIONS envvar support.

Improvements in DH_OPTIONS handling and DH_AUTO_OPTIONS envvar support.

* DH_AUTO_OPTIONS is like existing DH_OPTIONS, just only for dh_auto
  stuff. This also avoids "explosion" of separate DH_AUTO_* environment
  variables (i.e. exports in debian/rules) and encourages usage of dh_auto
  command line option names. DH_AUTO_OPTIONS is passed via "extra_args" to
  Dh_Lib::init() (API addition).
* When splitting options from DH_OPTIONS and its flavours, allow arguments
  to include whitespaces if they are escaped with backslash (\) (see
  split_options_string()). Document this in debhelper.pod.
* Short option for --buildsystem is -c (aka class).
* Provide API to cancel option specs from default debhelper options.
  It will be used in the feature.