[racket] 5.3's "mzc optimizer" log-debug, and log "facility" in general

Based on the discussion, plus some extra off-list discussion with Greg,
I've made the following changes:

 * The `log-error', etc. forms still treat a single subexpression as a
   literal string for the log message, but now they treat multiple
   subexpressions as arguments to `format'.

   This change balances backward compatibility, convenience, and
   interaction with the new `racket/format' library (which I'll post
   about on the dev list).

 * Add `define-logger', where

     (define-logger zoo)

   binds

     log-zoo-error
     log-zoo-warning
     ...

   that write to a logger that is created a child of `(current-logger)'
   with the name 'zoo.

   Unlike what I suggested before, the `define-logger' form doesn't
   create a parameter and it doesn't inspect environment variables.
   Those ideas were on the wrong track --- trying to filter at the
   producer side, instead of the consumer side of logging.

 * Changed `make-log-receiver' to accept a sequence of level symbols
   and name symbols. For example,

      (make-log-receiver logger 'debug 'GC 'error)

   receives 'debug level events from children of `logger' that are
   named 'GC, and it receives 'error events from all other loggers
   under `logger'.

   The logger name in a receiver affects `log-level?' queries in the
   way that you'd expect, which avoids unnecessary work on the producer
   side while keeping consumers in control.

 * Changed -W, -L, PLTSTDERR, and PLTSYSLOG to allow specifications
   like "debug at GC error", which is analogous to the `make-log-receiver'
   call above. That is, a level can be suffixed with "@" and a logger
   name to restrict the level specification to apply to a logger name.

   Note that you can use
     "debug none at optimizer"
   to see all debug messages except those from 'optimizer, since a
   level without a name applies only to loggers that are not otherwise
   mentioned.

   This change is based on Neil's suggestion, but with a syntax that I
   find more suggestive and more in parallel to `make-log-receiver'.

 * Changed the GC to log messages to a 'GC logger, the optimizer to log
   to an 'optimizer logger, and the futures subsystem to log to a
   'future logger.