ensure IPv4 is used with --icmp-timestamp

ensure IPv4 is used with --icmp-timestamp

The ICMP Timestamp message is defined for IPv4, but not for IPv6.

Before, combining --icmp-timestamp with an IPv6 address or a DNS
name for a dual-stacked target on a dual-stacked system resulted
in sending ICMPv6 Echo over IPv6.

Now, DNS names for dual-stacked hosts just work, i.e., fping uses
IPv4 with --icmp-timestamp.  Combining --icmp-timestamp with an
IPv6 address or an IP6-only DNS name now fails.