ping(8): Improve parameter validation

• Use strtonum(3) to simplify bounds checking of numeric parameters.
• Fix bounds checking when filling out packet data in "sweep" mode.

PR: 239974, 239977, 239978
Reported by: Neeraj <neerajpal09@gmail.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25622