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

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

For functions that return -1 on failure check exactly for -1 and not for
any negative number.

MFC after: 3 days

Use PJDLOG_ASSERT() and PJDLOG_ABORT() everywhere instead of assert().

MFC after: 3 days

Forgot to add event.c and event.h in r212038.

Pointed out by: pluknet <pluknet@gmail.com>
MFC after: 2 weeks
Obtained from: Wheel Systems Sp. z o.o. http://www.wheelsystems.com