Add a flag for -F/--foreground

[ronnie sahlberg]

I think it would be useful to run everything in the foreground so we can 

run it under gdb.

-f is already taken unfortunately by ublk.loop, but maybe we can add a

-F/--foreground for this.

I can add this if you want.

[Ming Lei]

After commit 4ac21f6 ("targets: run ublk.target in foreground"), the target is
always run in foreground by starting `./ublk.{target}`, and `ublk` just becomes
wrapper for starting `./ublk.{target}` in background.

If you think that isn't enough, we can add one argument to the `ublk`
executable.


Thanks,
Ming Lei