mavproxy_link: add "ping" command

[peterbarker]

issues TIMESYNC requests and prints results

A CubeOrange connected via USB:

ACRO> ping
ACRO> ping response: 3.075ms from=1/1 in-link=0 out-link=0

Multiple links to a SITL instance:

STABILIZE> link list
STABILIZE> 2 links
0: :14550
1: localhost:17000
ping
STABILIZE> ping response: 6.180ms from=1/1 in-link=0 out-link=0
ping response: 6.461ms from=1/1 in-link=0 out-link=1
ping response: 6.886ms from=1/1 in-link=1 out-link=0
ping response: 6.996ms from=1/1 in-link=1 out-link=1

Uses console:

[peterbarker]

... a link to a second simulation:

STABILIZE> link list
STABILIZE> 3 links
0: :14550
1: localhost:17000
2: localhost:18000

STABILIZE> 
STABILIZE> ping
STABILIZE> ping response: 4.781ms from=1/1 in-link=0 out-link=0
ping response: 5.253ms from=1/1 in-link=0 out-link=1
ping response: 5.598ms from=1/1 in-link=1 out-link=0
ping response: 5.761ms from=1/1 in-link=1 out-link=1
ping response: 18.062ms from=17/1 in-link=2 out-link=2

(second vehicle was Plane sim.... longer loop times == higher average latency)