dhcpcd: Accept an ACK after a NAK

When a NAK is received, wait for a response interval before
acting on it.  In doing so, this allows for networks that
may have duelling DHCP servers which both consider themselves
authoritative on a network.  Since other DHCP clients also
act in a similar manner, issues of this sort end up undetected
and network administrators thus consider this situation nominal.

BUG=chromium:384897
TEST=New network_DhcpNak test subcase in CL:208214

Change-Id: I8ca5daa5eb1f115066d024f4982ac7ebb8a6b54a
Reviewed-on: https://chromium-review.googlesource.com/208207
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
2 files changed