Checking for another DHCP server doesn't work properly on OpenBSD

[EugeneOne1]

In #3225 we've added an incomplete support for DHCP on OpenBSD. Checking for other DHCP servers in the network still doesn't work properly in some circumstances.

It seems that net.ListenPacket binds to the unspecified IP address with the specified port and ignores packets that were read by any other listener of the same port. Setting socket options SO_REUSEPORT and SO_REUSEADDR also didn't help.

Perhaps, we may peek some implementation details in github.com/insomniacslk/dhcp/dhcpv4/nclient4.

[EugeneOne1]

After a bit of investigating, the problem seems to require more sofisticated raw socket manipulation. As a workaround, we may recommend disabling the competing processes (e.g. dhclient, dhcpd, etc) manually on OpenBSD. We'll consider adding this to the documentation until a better solution appears.