mavproxy_rc: add status command

[peterbarker]

rc 3 1000
MANUAL> rc 4 65535
MANUAL> rc status
MANUAL> 
 1: 0      (no override)
 2: 0      (no override)
 3: 1000
 4: 65535      (ignored)
 5: 0      (no override)
 6: 0      (no override)
 7: 0      (no override)
 8: 0      (no override)
 9: 0      (no override)
10: 0      (no override)
11: 0      (no override)
12: 0      (no override)
13: 0      (no override)
14: 0      (no override)
15: 0      (no override)
16: 0      (no override)
17: 0      (no override)
18: 0      (no override)