Use internal insead of private for config

Use internal insead of private for config

The command framework used by k5start and krenew has a shared
configuration plus internal configuration structs for each program
that hold their own options.  The latter were previously called
private, but this is a C++ reserved word that causes problems with
various tools (such as clang-format) that understand both C and
C++.  Rename those structs to internal.