(1) Change the mapping from zLOG levels to logging levels to use
    custom intermediary levels 15 and 5 for BLATHER and TRACE, rather
    than using a confusing skewed mapping.

(2) In the ZConfig datatype definition for a logging level, added
    'blather' and 'trace' to the level names, added 'warning' as an
    alias for 'warn', change 'all' to mean 1, and add 'notset' to mean
    0.  The semantics of NOTSET are very different than those of 1;
    setting a logger's level to NOTSET searches for a parent logger
    with a nonzero level.  The root of all loggers is initialized with
    WARN as its level, so setting the level to NOTSET effectively sets
    it to WARN rather than to the most inclusive level!

(3) In the schema, change the default level for handlers to notset, so
    they use their logger's level; the default level for the logger is
    set to info, corresponding to the old default for
    STUPID_LOG_SEVERITY.