another round of debbisect improvements

• provide a pty to subprocess and capture its output in real-time, enabling even things like progress bars if run with --verbose
• print the snapshot.d.o http status if it's not the expected one
• further throttling seems to be needed for snapshot.d.o
• add --ignore-cached-results option
• document the earliest possible snapshot.d.o timestamp
• check for mmdebstrap version if --depends or --qemu was passed
• use the maybe-jessie-or-older and maybe-merged-usr hooks to do the right thing depending on the snapshot
• allow running with --qemu but without --depends
• run with --skip=check/signed-by for a small speedup