trace: deprecate BEESLOGTRACE, align trace logs with exception notices

trace: deprecate BEESLOGTRACE, align trace logs with exception notices

Exceptions were logged at level NOTICE while the stack traces were logged
at level DEBUG.  That produced useless noise in the output with `-v5`
or `-v6`, where there were exception headings logged, but no details.

Fix that by placing the exceptions and traces at level DEBUG, but prefix
them with `TRACE:` for easy grepping.

Most of the events associated with BEESLOGTRACE either never happen,
or they are harmless (e.g. trying to open deleted files or subvols).
Reassign them to ordinary BEESLOGDEBUG, with one exception for
unrecognized Extent flags that should be debugged if any appear.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>