With the new "only print if sth happens" way, sometimes stats is
just silent forever. This happens if we are master or if no slave
is active (well, and during initial setting up the wr link).

So now typing "stat" or "stat on" will turn on stats and print a line
of statistics, every time -- or the line "statistics now off".
This happens by changing the internal status of monitor_ppsi.c,
now exported to the cmd_stat.c.

This is the result on a master system:

   wrc# stat
   lnk:1 rx:0 tx:3 lock:1 sv:0 ss:'' aux:1 sec:5 [...]
   wrc#
   wrc#
   wrc# stat
   statistics now off
   wrc#
   wrc#
   wrc# stat
   lnk:1 rx:0 tx:11 lock:1 sv:0 ss:'' aux:1 sec:8 [...]
   wrc#
   wrc#

And this is what happens when we become slave:

   wrc# mode slave
   Locking PLL
   wrc# ptp start
   Slave Only, clock class set to 255
   wrc# lnk:1 rx:57 tx:60 lock:1 sv:1 ss:'SYNC_SEC' aux:1 sec:49 [...]
   lnk:1 rx:70 tx:64 lock:1 sv:1 ss:'SYNC_NSEC' aux:1 sec:1444901411 [...]
   lnk:1 rx:77 tx:66 lock:1 sv:1 ss:'SYNC_PHASE' aux:1 sec:1444901414 [...]
   lnk:1 rx:83 tx:68 lock:1 sv:1 ss:'WAIT_OFFSET_STABLE' aux:1 sec:1444901416 [...]
   lnk:1 rx:87 tx:69 lock:1 sv:1 ss:'WAIT_OFFSET_STABLE' aux:1 sec:1444901417 [...]
   lnk:1 rx:90 tx:70 lock:1 sv:1 ss:'TRACK_PHASE' aux:1 sec:1444901418 [...]
   lnk:1 rx:94 tx:71 lock:1 sv:1 ss:'TRACK_PHASE' aux:1 sec:1444901419 [...]
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>