MFC r336551:
Add missing dtrace probes for received UDP packets.

Fire UDP receive probes when a packet is received and there is no
endpoint consuming it. Fire the probe also if the TTL of the
received packet is smaller than the minimum required by the endpoint.

Clarify also in the man page, when the probe fires.

Reviewed by: dteske@, markj@, rrs@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D16046

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix example code rendering, \n needs escaping to show up.

PR: 203536
Submitted by: Fabian Keil

Rename the dtrace-* man pages to dtrace_* for consistency with other
subsection man pages (e.g. geom_*, mac_*, snd_*).

Add manual pages for the io, ip, proc, sched, tcp and udp DTrace providers.
The format of these pages is somewhat experimental, so they may be subject
to further tweaking.

Differential Revision: https://reviews.freebsd.org/D2170
Reviewed by: bcr, rpaulo
MFC after: 2 weeks