sbig-snap: add --preview option for ds9

After writing FITS file, spawn xpaset command to tell ds9 to
display the file.  The file is provided to xpaset on its stdin.

fixes issue #14
sbig: set XPA_NSINET if configured

This allows for sub-commands to transparently get remote ds9 access
via xpaset/xpaget.  For this to work ds9 needs two things:

1) it needs to be told to bind to an address other than localhost.
One way is to set this environment variable before starting ds9:
   export XPA_HOST=<ipaddr>

2) it needs to be told to allow specific hosts to connect
One way is to create $HOME/acls.xpa with the line:
   DS9:ds9 <ipaddr> +
doc: describe ds9 preview in README.md