Update to work with libcontainer

Makes pipework work with the new docker native exec driver (libcontainer).
Use pid from docker inpsect

Since docker returns a pid for the container use that instead of doing all the other hackery.
Merge pull request #41 from cpuguy83/patch-2

Update to work with libcontainer
Adding functionality to find any available dhcp client(dhclient,udhcpc,
or dhcpcd) for use in cases where udhcpc may not be readily available.
Also a few checks to not run mkdir or rm unless necessary.
Merge pull request #56 from bcicen/master

Adding functionality to find any available dhcp client
Currently, if arping is found, the script will always exit with exit
code 1. This is because arping sends a ping to cause other devices to
notice the new MAC address, but this ping is not expected to be
returned. However, this means that the exit code from the arping line is
always 1. When running manually this isn't noticeable as it's the last
action performed, but it plays serious havoc when calling pipework from
another program. The fix is to OR the exit code with true.
missing dev parameter to set mac  address
Merge pull request #64 from codingtony/master

missing dev parameter to set mac  address
Merge pull request #59 from jefferai/master

Don't always exit with status 1 when arping is found