tree-wide: change --kill-who to --kill-whom

tree-wide: change --kill-who to --kill-whom

getopt allows non-ambiguous abbreviations, so backwards-compat is maintained, and
people can use --kill-who (or even shorter abbreviations). English is flexible,
so in common speach people would use both forms, even if "whom" is technically
more correct. The advantage of using the longer form in the code is that we
effectively allow both forms, so we stop punishing people who DTGCT¹, but still
allow people to use the spoken form if they prefer.

1. Do the gramatically correct thing