acme: implement 'scratch' ctl command (thanks Drew DeVault)

acme: implement 'scratch' ctl command (thanks Drew DeVault)

The new command marks the target window as a scratch window -- a window
whose state cannot be "dirtied" by changes made to its body, therefore
avoiding warnings about unsaved changes when deleting the window or
exiting acme.

Existing examples of scratch windows are error, directory, and guide
windows, whose scratchness is set internally.

With the new command users and programs alike can create their own
scratch windows.  This is put to use in acme's own win(1).