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

Merge r256514:
- While we are spreading the counter(9) across network stack, more userland
tools would need to know about the counter_u64_t type. Allow to include
sys/counter.h from userspace.

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

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

Add several macros to help migrate statistics structures to PCPU counters.

Merge from projects/counters: counter(9).

Introduce counter(9) API, that implements fast and raceless counters,
provided (but not limited to) for gathering of statistical data.

See http://lists.freebsd.org/pipermail/freebsd-arch/2013-April/014204.html
for more details.

In collaboration with: kib
Reviewed by: luigi
Tested by: ae, ray
Sponsored by: Nginx, Inc.