Split PruneFreezeParams initializers to one field per line

Split PruneFreezeParams initializers to one field per line

This conforms more closely with the style of other struct initializers
in the code base. Initializing multiple fields on a single line is
unpopular in part because pgindent won't permit a space after the comma
before the next field's period.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reported-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://postgr.es/m/87see87fnq.fsf%40wibble.ilmari.org