client: consider config in ifstatus, exit code rework, copyright/license updates

-   Moved ifup,ifdown,ifcheck to separate .c and .h files.

-   Updated copyright/license in all client files.

-   Moved ifstatus to separate files, changed to use fsm to
    consider current config files and refactored to split the
    status checks using new ifcheck helpers from showing info.

-   Reworked "ifstatus all" to reduce the status to well-known
    LSB 0..4 codes considering mandatory devices and consistent
    use of >150 codes [LSB app range] in a single interface run.

-   Adpted the ifup compat and systemv network init scripts.