Allow a EVFILT_TIMER kevent to be updated.

If a timer is updated (re-added) with a different time period
(specified in the .data field of the kevent), the new time period has
no effect; the timer will not expire until the original time has
elapsed. This violates the documented behavior as the kqueue(2) man
page says (in part) "Re-adding an existing event will modify the
parameters of the original event, and not result in a duplicate
entry."

This modification, adapted from a patch submitted by cem@ to PR214987,
fixes the kqueue system to allow updating a timer entry. The
kevent timer behavior is changed to:

• When a timer is re-added, update the timer parameters to and re-start the timer using the new parameters.
• Allow updating both active and already expired timers.
• When the timer has already expired, dequeue any undelivered events and clear the count of expirations.

All of these changes address the original PR and also bring the
FreeBSD and macOS kevent timer behaviors into agreement.

A few other changes were made along the way:

• Update the kqueue(2) man page to reflect the new timer behavior.
• Fix man page style issues in kqueue(2) diagnosed by igor.
• Update the timer libkqueue system test to test for the updated timer behavior.
• Fix the (test) libkqueue common.h file so that it includes config.h which defines various HAVE_* feature defines, before the #if tests for such variables in common.h. This enables the use of the actual err(3) family of functions.
• Fix the usages of the err(3) functions in the tests for incorrect type of variables. Those were formerly undiagnosed due to the disablement of the err(3) functions (see previous bullet point).

PR: 214987
Reported by: Brian Wellington <bwelling@xbill.org>
Reviewed by: kib
MFC after: 1 week
Relnotes: yes
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D15778