pf tests: Remove nargs from single arguments

[vegeta_tuxpowered.net]

Summary

Using nargs=1 causes arguments to produce lists from which we must then
get the real value. The same applies to defaults, the must be provided
as arrays too.

Remove nargs=1 so that values and defaults can be accessed directly.

[kp]

This seems to include a move to sys/netpfil/common/pft_ping.py, but not the makefile changes that would require. I assume the move is not intended to be part of this patch?

Other than that this looks good, so when my test run completes I intend to commit this without the move.