Support for the "install_headers" command:

Support for the "install_headers" command:
  * 'headers' entry added to all the install schemes
  * '--install-headers' option added
  * 'install_headers' added to 'sub_commands'
  * added 'dist_name' to configuration variables (along with a few
    others that seem handy: 'dist_version', 'dist_fullname', and
    'py_version'
  * in 'finalize_unix()', make sure 'install_headers' defined if
    user specified 'install_base' and/or 'install_platbase'
  * added 'has_headers()'
  * a few other small changes