GIDs

- Compile without /dev/kmem access.  This requires a small patch which
  opens libkvm in a dummy mode which uses sysctls to implement most of
  its interfaces rather than /dev/kmem access.  This way we can drop the
  dependency on /dev/kmem without rewriting existing code.
- Add a new snmpd user.  Configure snmpd to drop privileges once it's
  finished initialization.
- Remove the JAIL option.  Now that snmpd avoids using /dev/kmem,
  there's no need to have a special mode for running snmpd in jails.

The patch has been proposed upstream here:
https://sourceforge.net/p/net-snmp/mailman/net-snmp-coders/thread/ZjEwNV5BiTOQ-Adi%40nuc/#msg58766857

Approved by:	zi
Sponsored by:	Klara, Inc.
Sponsored by:	Stormshield
Differential Revision:	https://reviews.freebsd.org/D45031