Support for nested program

Hi,

Although probably not often used, it might be interesting in the future that CCF support nested programs.

At this time, it is not a priority for us, just an opportunity to complete the functionalities of CCF.
But maybe in the future we will use the COBOL functionality of nested programs because we are in the process of reviewing our entire COBOL development framework to take advantage of the latest developments in the IBM Enterprise COBOL compiler.
COBOL V6R4, supports user-defined functions, maybe we'll implement them through nested programs, (which also assumes that CCF supports user-defined functions).