Follow the declared behaviour that specifies server string format in
bsnmpclient(3).

snmp_parse_server() function accepts string where some fields can be
omitted: [trans::][community@][server][:port]

"trans" field can be "udp", "udp6", "dgram" and "stream".
"community" can be empty string, if it is omitted, the default value
will be used. For read_community it is "public", for write_comminity
it is "private". "server" field can be hostname, IPv4 address or IPv6
address. IPv6 address should be specified in brackets "[]".
If port is omitted, the default value "snmp" will be used for "udp"
and "udp6" transports. So, now for bsnmpget(1) and bsnmwalk(1) it is
not required to specify all fields in argument of '-s' option. E.g.

1. bsnmpget -s 127.1 sysName.0
2. bsnmpget -s "udp::127.1" sysName.0
3. bsnmpget -s "udp::public@127.1" sysName.0
4. bsnmpget -s "udp::public@127.1:161" sysName.0
5. bsnmpget -s "udp::[::1]" sysName.0
6. bsnmpget -s "udp6::[::1]" sysName.0
7. bsnmpget -s "[fe80::1%lo0]" sysName.0

PR: 236664
Reported by: olivier
MFC after: 1 month