[PATCH] Two TODOs

Attached patch implements the following TODOs:

o  Add to the 'M'enu:
     '!'  (GLOBAL_SHELL_ESCAPE)

and

o  Additions to the thread menu:
     'D'  (cancel article)

ShellEscape ('!') is possible from config 'M'enu, scope menu and
attributes menu now.

'D' (Cancel) is now available from group and thread level.

cancel_article() needs a valid pgart, so art_open() and art_close() are
necessary to populate (and clear) pgart. art_open() calls, among other
things, cook_article(), which is unnecessary in this case. I am not sure
whether this overhead is acceptable here. On the other hand, cancelling
an article is not the usual task, so this might be OK.

Dennis