Save data to cross-pol specific datasets

Now each cross pol product (XX, YY, XY, and YX) are stored in separate
datasets.  The dataset name format is "zen.JD.FRAC.PP.uv", where JD.FRAC
is the Julian Day with FRACtional part and PP is the (lowercase)
cross-pol name.
Format packet stats to be more readable

The packet stats are now printed 8 XIDs to a line.  This makes for
better readability in general and, for the XID/XHOST assignments of the
PAPER 256 input correlator, it puts each X host's packet stats in a
column.
Increase requested SO_RCVBUF to 257 MiB

257 MiB is enough to buffer one entire dump of a 256 input 1024 channel
correlator (assuming 8 bytes per complex value).