Fix the UDP tests for dtrace.

The code imported from opensolaris was depending on ping supporting
UDP for sending probes. Since this is not supported by ping on FreeBSD
use a perl script instead.
The remote test requires the usage of ksh93, so state that in the
sheband.
Enable the local test, but keep the remote test disabled, since it
requires a remote machine on the LAN.

Reviewed by: markj@, gnn@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D16268