Description:
As viewed from https://github.com/archlinux/svntogit-packages/blob/fa3fdddca283031f4c5ccb68a091f8e25fc7b32f/trunk/snmpd.service , the current version of `snmpd.service` defines the below startup directive:

```
[Service]
ExecStart=/usr/bin/snmpd -p /run/snmpd.pid
```

This causes `snmpd` service to listen to the wildcard IPv4 address only (`0.0.0.0:161`), as it is the default `snmpd` behavior when no other listening address are specified on its command line.

```
# ss -puln sport = 161
State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
UNCONN 0 0 0.0.0.0:161 0.0.0.0:* users:(("snmpd",pid=1585,fd=7))
```

For better compatibility with IPv6-enabled hosts, I would like to request a modification on the `snmpd.service` unit which would allow `snmpd` service to listen to the wildcard IPv6 address (`[::]:161`) as well. The startup directive should be defined as below:

```
[Service]
ExecStart=/usr/bin/snmpd -p /run/snmpd.pid udp: udp6:
```

```
# ss -puln sport = 161
State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
UNCONN 0 0 0.0.0.0:161 0.0.0.0:* users:(("snmpd",pid=223276,fd=7))
UNCONN 0 0 [::]:161 [::]:* users:(("snmpd",pid=223276,fd=8))
```

Additional info:
* package version(s) - net-snmp 5.9.1-3
* config and/or log files etc. - (not applicable)
* link to upstream bug report, if any. - (not applicable)

Steps to reproduce:
Install `net-snmp` package from `extra` repository, and start the `snmpd.service`.