Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r233628, r234598, r235229, r235831, r226986.

Add software PMC support.

New kernel events can be added at various location for sampling or counting.
This will for example allow easy system profiling whatever the processor is
with known tools like pmcstat(8).

Simultaneous usage of software PMC and hardware PMC is possible, for example
looking at the lock acquire failure, page fault while sampling on
instructions.

Sponsored by: NETASQ

MFC: r231871

Switch the license boilerplates to our standard one.

Advantages:
- Reduces the number of different license versions in the tree
- Eliminates a typo
- Removes some incorrect author attributions due to c/p
- Removes c/p error potential for future pmc manpages

Approved by: jkoshy, gnn, rpaulo, fabient (copyright holders)

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

mdoc: order prologue macros consistently by Dd/Dt/Os

Although groff_mdoc(7) gives another impression, this is the ordering
most widely used and also required by mdocml/mandoc.

Reviewed by: ru
Approved by: philip, ed (mentors)

Fix typos.

Fix typos.

Reported by: Harald Servat <redcrash at gmail dot com>

Document the fact that some Core2 family CPUs lack fixed-function counters.

Correction: these PMCs do not support a "umask" modifier.

Tweak -mdoc usage.

Fix typos, document UMASK values.

Document the alternate event names supported for "architectural" PMC events.

Add manual pages for performance measurement counters present in
Intel Atom(tm), Core(tm) and Core2(tm) CPUs.