meta-openembedded: net-snmp: systemd support

Backported systemd-related commits from net-snmp's master branch (full
description in the systemd-support.patch file).

.service files slightly adapted from Fedora's.

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>