Attempts are spaced out with exponential backoff, cut at 10 minutes per
attempt.
Also avoid trying to acquire an IP on interfaces that aren't up.
Fixes#6126.
Fixes#6125.
Real DHCP servers might decide to send the DHCPAck directly to the
specified ciaddr instead of as a unicast or multicast, resulting in
the ack being ignored by the network adapter when we are requesting
a new IPv4 address instead of renewing an existing lease, as the
yiaddr (and as a result the ciaddr) is set to the offered address in
that case instead of the current ip address.
Some real DHCP servers dont set the siaddr field in the DHCPOffer to
their IPv4 (and instead leave it blank - 0.0.0.0), which results in
the server assuming the DHCPRequest is not directed at him when it
has the ServerIdentifier option attached that specifies 0.0.0.0 as
the targeted server. So instead we just omit the option and let the
DHCP servers decipher the target themselves based on the requested IP.
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
