enhanced 5250 protocol patch

I've been working on adding support for the enhanced 5250 protocol to
lib5250.  So far my work has been changing that connection negotiation to
let the remote host know we support the enhanced protocol and handling the
CREATE WINDOW Structured Field, REMOVE GUI WINDOW Structured Field, and
REMOVE ALL GUI CONSTRUCTS Structured Field commands.

These new features require +enhanced to be in the user's ~/.tn5250rc file
to be enabled.  Without it, no changes occur so it shouldn't affect anyone
(unless they change their .tn5250rc file, obviously).

Interestingly, with my patch cursor progression works correctly for
continuous fields.

With this patch a new window handling method is required when +enhanced is
enabled.  x5250 version 0.1.8 shows an incomplete implementation of the
new method.  Compiling x5250 0.1.8 with -DEHANCED defined will enable the
new handlers.

This patch adds a new window structure to track every window created with
the CREATE WINDOW Structured Field.

This patch is against current CVS.

James Rich

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