from netsnmp.20 to netsnmp.30

[Pavel Timofeev]

since we have net-snmp 5.7 in ports it would be right to correct some ports
from netsnmp.20 to netsnmp.30

[root at monitor /usr/ports]# grep -R netsnmp.20 *
french/plgrenouille/Makefile:LIB_DEPENDS=
netsnmp.20:${PORTSDIR}/net-mgmt/net-snmp
security/libfwbuilder/Makefile:
netsnmp.20:${PORTSDIR}/net-mgmt/net-snmp
sysutils/rsyslog5-devel-snmp/Makefile:LIB_DEPENDS=
netsnmp.20:${PORTSDIR}/net-mgmt/net-snmp