cmd-line flag for -diag only mode

If cbft is happily running, then a user/client can send a REST /api/diag request to get tons of diagnostic response info.

However, if cbft isn't running... perhaps it's crashed and can't restart... then an interesting feature might be to run in a "grab diagnostic output only" mode, like "./cbft -diag"

That would just grab any info it can from any leftover local dataDir, and perhaps try to contact the cfg server, but then soon emit whatever it can, best effort, and exit.