dhcpcd improvements

1. Redirect stderr to stdout when running `which` to avoid noise every time `pipework` tries to discover the appropriate DHCP client.

        which: no udhcpc in (/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/home/marc/bin:/usr/local/go/bin)

2. `dhcpcd` first execution will prevent subsequent executions to succeed by locking the pid file associated to `CONTAINER_IFNAME`

3. Disable `dhcpcd` run hooks. Hooks are executed by the host.

4. Gain 5 seconds by not claiming the DHCP assigned IP address.
Will it always be that path? And what if I really have a DHCP client for that interface ... ?