Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

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

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 after: 1 month

Remove useless Ta macro.

Remove superfluous paragraph macro.

Update manual pages for MIPS-related CPUs:

- Rename pmc.mips to pmc.mips24k since it covers just one CPU,
no whole architecture
- Add documetnations for Octeon's PMC counters
- Remove CAVEATS section from pmc.mips24k page: PMC for MIPS supports
sampling now.