[software/vdif_corr] Fixes to the plotting part of VDIF corr.

Now it won't complain about xlim not existing. Also, when given a single file
as input it will only show the auto-correlation.
[software/vdif_corr] Added a frame limiting option.

For large files you can limit the number of frames correlated with the
-n option. This took over the FFT size short option. To set the FFT size
use the long option --nfft.