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

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)
MFC after: 1 week

Add a flush of the current PMC log buffer before displaying the next top.

As the underlying block is 4KB if the PMC throughput is low the measurement
will be reported on the next tick. pmcstat(8) use the modified flush API to
reclaim current buffer before displaying next top.

MFC after: 1 month

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)

- Describe function pmc_init() in its own manual page.
- Describe functions pmc_configure_logfile(), pmc_flush_logfile() and
pmc_writelog() in their own manual page.