ping(8): man page cleanup

ping(8): man page cleanup

* Appease mandoc -T lint and igor

* Use example.com for documentation

* Update the IPv4 TTL section.
  Update the IPv4 TTL section specifically for FreeBSD.
  FreeBSD changed the default TTL to 64 in
  5639e86bdd7ea151958776264bf5a67e60a54d68.  NetBSD and OpenBSD still
  use 255.  Remove some references of extinct operating systems.

Reviewed by:	gbe (manpages), asomers
Pull Request:	https://github.com/freebsd/freebsd-src/pull/630

(cherry picked from commit 8eb4df948711166a438f4111f7069a412d1456bd)