ping: Fix an unsigned integer overflow (D38470 alternate take)

[jlduran]

Summary

The function pr_pack() prints out a packet, if the IP packet contains options, these are printed as well.
Commit 46d7b45a267b3d78c5054b210ff7b6c55bfca42b introduced an integer overflow bug, by changing hlen from int to uint8_t.
Revert this part of the code to the way it was before by creating an int type variable optslen, to be decremented instead of hlen.

Fixes: 46d7b45a267b3d78c5054b210ff7b6c55bfca42b

Test Plan

DISCLAIMER: My preferred route would be to implement D38431, however for reasons detailed in that review, we'll resort to fixing what we currently have.

[cy]

This is certainly more complex than simply changing hlen back to a signed int.

[cy]

See https://reviews.freebsd.org/D38744 for a simpler alternative.