net/fping: fix WITHOUT_IPV6 mode

If one builds and uses net/fping version 4.2 with option IPV6 disabled,
the command fping -4 always fails with wrong error message:

fping: can't specify both -4 and -6

This makes it impossible using fping probes with SmokePing
as it runs fping with -4 option. This change fixes this.
Also, do not install symlinks for fping6 and manual in this case.

PORTREVISION not bumped as default build is not affected.

PR: 238327
Approved by: jharris@widomaker.com (maintainter timeout, 2 weeks)