MFC 339620: Add a "live" mode to ktrdump.

Support a "live" mode in ktrdump enabled via the -l flag. In this
mode, ktrdump polls the kernel's trace buffer periodically (currently
hardcoded as a 50 millisecond interval) and dumps any newly added
entries. Fancier logic for the timeout (e.g. a command line option or
some kind of backoff based on the time since the last entry) can be
added later as the need arises.

Note that this does not include the capsicum fixes from the original
commit since ktrdump in stable/11 is not capsicum-ized.

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

Change ktrdump to use the more standard -M/-N flags to specify the path
to a crash dump and kernel, respectively. The existing -m/-e flags are
still supported for backwards compatiblity but are no longer documented.

Requested by: np
MFC after: 2 weeks

use .Mt to mark up email addresses consistently (part3)

PR: 191174
Submitted by: Franco Fichtner <franco at lastsummer.de>

Add -H flag to print thread id.

Markup and wording fixes.

Approved by: re (blanket)

Document the '-i' option which allows the user to specify a ktr events
log file.

mdoc(7) fixes.

Add a "-r" flag to ktrdump(1) to print relative timestamps when used
with "-t" rather than absolute timestamps. This allows the reader
to get a better sense of latency between events, such as time to
schedule an interrupt thread from time the interrupt occurred. Assert
a copyright on ktrdump.c since I seem to be modifying it more than I
thought.

Add a quiet mode to ktrdump(1): if the "-q" flag is used, don't print
the pretty text header on top of the output. Simplifies feeding the
results of tracing into a script for mechanical processing.

The .Nm utility

mdoc(7) police: fixed typo, minor markup nits.

Add a man page.

Submitted by: davidc