check_snmp IP address space options

[eponymous alias]

In the monitoring plugins 2.2 release, the "check_snmp --help" output
says this:

 -4, --use-ipv4
    Use IPv4 connection
 -6, --use-ipv6
    Use IPv6 connection

but the code says otherwise about how the long options are named:

                {"ipv4", no_argument, 0, '4'},
                {"ipv6", no_argument, 0, '6'},

This discrepancy can cause a lot of confusion.

Looking around at the other plugins that have similar arguments:

    check_by_ssh
    check_dig
    check_fping
    check_http
    check_ldap
    check_ntp
    check_ntp_peer
    check_ntp_time
    check_ping
    check_smtp
    check_ssh
    check_tcp

their code all supports the full --use-ipv4 and --use-ipv6 arguments, 
and not the --ipv4 and --ipv6 forms.  So for the sake of consistency,
the code in check_snmp.c should be modified to support what the help
message says.  Perhaps for backward compatibility, it might also be
useful to continue supporting the --ipv4 and --ipv6 forms with
check_snmp, but I will leave that for others to decide.