OpenBSD change from select() to poll(), due to descriptor limits in the former.

This is an adaptation of the following OpenBSD revisions on to the
mrouted-3.9beta3+IOS12 baseline this mrouted is based on.

- defs.h r1.12
- main.c r1.20
- rsrr.c r1.11

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
OpenBSD inspired deregister.
OpenBSD UNIX 2038 Audit

The DVMRP protocol generation number can work beyond 2038 because it is
unsigned.  Cast it specifically, with a comment, to make this more
clear.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>
Fix type mixup in int/u_long when there should be time_t
OpenBSD: Remove old broken SNMP support.

Follow suite with OpenBSD in removing unused/uncompilable SNMP support.
Highly unlikely anyone could ever have used this.  Removing for improved
readbility.  Future support should be AgentX anyway.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>