sbin/ping: allow normal users to specify larger packets

Only super-user could specify a packet size larger than the default 56 bytes.
This restriction was added by Matt Dillon in 1998 during the BEST days [0].
This restriction doesn't exist in ping IPV6 or on NetBSD, OpenBSD and Linux.

UMS [1] uses this feature to estimate the client's bandwidth to optimize the
streaming experience.

[0] DFGit 526f06b278d9252add168aa18b60242c08771165
[1] UMS: https://github.com/UniversalMediaServer/UniversalMediaServer

Obtained from: DragonFlyBSD
Differential Revision: https://reviews.freebsd.org/D45774