Fix several bugs in input/output fromat

Thanks to vk1et for these.

1) Correct for additional timestamp langth in raw output buffer when
using mlat mode

2) Don't output a timestamp when the message has been received from a
remote site (the internet). This is to avoid upsetting MLAT because
there is an indeterminate delay between reception at the remote site and
subsequent message arrival in the local dump1090 instance.

3) Allow @ character for raw data input with timestamp, and correctly
calculate the length.