Add ping_monitor() to mdadm --wait

Add ping_monitor() to mdadm --wait

The action we are waiting for may not be complete until the monitor has
had a chance to take action on the result.

The following script can now remove the device on the first attempt,
versus a few attempts with the original Wait():
#!/bin/bash
#export MDADM_NO_MDMON=1
export IMSM_DEVNAME_AS_SERIAL=1
./mdadm -Ss
./mdadm --zero-superblock /dev/loop[0-3]
echo 2 > /proc/sys/dev/raid/speed_limit_max
./mdadm --create /dev/imsm /dev/loop[0-3] -n 4 -e imsm -a md
./mdadm --create /dev/md/r1 /dev/loop[0-3] -n 4 -l 5 --force -a mdp
./mdadm --fail /dev/md/r1 /dev/loop3
./mdadm --wait /dev/md/r1
x=0
while  ! ./mdadm --remove /dev/imsm /dev/loop3 > /dev/null 2>&1
do
        x=$((x+1))
done
echo "removed after $x attempts"
./mdadm --add /dev/imsm /dev/loop3

Include 2 small cleanups:
* remove the almost open coded fd2devnum() in Wait() by introducing a
  new utility routine stat2devnum()
* teach connect_monitor() to parse the container device from a subarray
  string

Signed-off-by: Dan Williams <dan.j.williams@intel.com>