sbin: build ping if at least one of INET & INET6 is enabled

It does not build (and serves no purpose) if neither is true (i.e.,
building WITHOUT_INET and WITHOUT_INET6). Also add an explicit error
in ping to make this case clear.

PR: 260082
Sponsored by: The FreeBSD Foundation

(cherry picked from commit a4ef9e58bc0c07110a54ba0fa88eb118c5377e6f)

OptionalObsoleteFiles: remove ping with INET & INET6 disabled

Reported by: kevans
Fixes: a4ef9e58bc0c ("sbin: build ping if at least one of...")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 0179739a0096c62cb3c9665d68246046255ab849)