Add a one-shot callout facility to db_printf() that executes the registered

Add a one-shot callout facility to db_printf() that executes the registered
callout when a specified number of lines have been output.  This can be
used to implement pagers for ddb commands that output a lot of text.  A
simple paging function is included that automatically rearms itself when
fired.

Reviewed by:	bde, julian