Embedded shell cmdbox-sh with completion

[rwxrob]

As cmdbox matures the obvious next step is to create a standard cmdbox-sh package that contains the code from BusyBox (ash) and modify it to use the minimal COMP_LINE completion approach by default. All we have to do is trap the tab keystrokes and recursively call the same multicall binary after setting COMP_LINE. POSIX (ash) with minimal tab completion is literally the Holy Grail of FROM scratch container development.