network-manager: adjust dhclient and dhcpcd directories

dhclient and dhcpcd were moved from /usr/sbin to /sbin for busybox-less
SysV-style init with Debian utilities, so adjust accordingly.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>