net-mgmt/net-snmp: Provide an option for snmptrapd to drop privs

As with snmpd, we can run snmptrapd with reduced privileges, which is
certainly desirable since snmptrapd's main function is to receive SNMP
traps and log them somewhere.

Approved by: zi
Sponsored by: Klara, Inc.
Sponsored by: Stormshield