dhcpcd-6.8.2: switch user after starting dhcpcd-6.8.2

Have dhcpcd-6.8.2 switch itself to the dhcp uid and gid after
spawning. This assumes that dhcpcd-6.8.2 is started as root,
which is current the case when shill spawns it.

BUG: 24408645
TEST: dhcpcd-6.8.2 builds and is started successfully by shill.
Change-Id: Ia14c31ddd7ac06cc19f0541ae3c7bf0f019447ce
1 file changed