ping: Unify ping/ping6 statistics section

This is a first step towards a unification/simplification of ping/ping6
(internally). The end goal is to produce a standardized user-facing
output.

Before (ping6):

PING6(56=40+8+8 bytes) 2001:db8::1 --> 2001:db8::2
16 bytes from ::1, icmp_seq=0 hlim=64 time=0.168 ms
16 bytes from ::1, icmp_seq=1 hlim=64 time=0.068 ms

--- 2001:db8::2 ping6 statistics ---
round-trip min/avg/max/std-dev = 0.068/0.118/0.168/0.050 ms

After (ping6):

PING(56=40+8+8 bytes) 2001:db8::1 --> 2001:db8::2
16 bytes from ::1, icmp_seq=0 hlim=64 time=0.168 ms
16 bytes from ::1, icmp_seq=1 hlim=64 time=0.068 ms

--- 2001:db8::2 ping statistics ---
round-trip min/avg/max/stddev = 0.068/0.118/0.168/0.050 ms

This has the nice side-effect of adding units to SIGINFO's statistics,
as printing numbers without units may not be of much help. Also
mentions the fact that these times are round-trip.

Before (ping/ping6 SIGINFO):

2/2 packets received (100.0%) 0.068 min / 0.118 avg / 0.168 max

After (ping/ping6 SIGINFO):

• <ipv4/ipv6 address> ping statistics --- 2 packets transmitted, 2 packets received, 0.0% packet loss round-trip min/avg/max/stddev = 0.068/0.118/0.168/0.050 ms

In the case of a SIGINFO, the output will be printed to stderr, for both
ping and ping6.

Reviewed by: markj
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/863
Differential Revision: https://reviews.freebsd.org/D39126

(cherry picked from commit 03d4d1c778091a08277d070185a9c60c7a6d57e0)