#2 Debug mode: print issued commands to stdout

[Ranieri Argentini]

Apply this patch to add a "-d" option that prints all
commands issued to the OS to stdout.

Might conflict with previous "interface needs to be up"
patch, but is trivially integrated.

We need separate system() and `backtick` handling to
make sure return values survive intact from system() if
needed.

[Ranieri Argentini]

Patch