ping6: Rename options for better consistency with ping
Now equivalent options have the same flags, and nonequivalent options have
different flags. This is a prelude to merging the two commands.
Submitted by: Ján Sučan <firstname.lastname@example.org>
Sponsored by: Google LLC (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21345