Logging: Documentation and minor cleanups of headers.

Logging: Documentation and minor cleanups of headers.

I have written complete documentation for the logging module. In the process,
I have performed a couple of minor cleanups:

   o  Internal declarations were moved to ucw/log-internal.h.
   o  Defines for "all" and "nothing" values have been removed, the customary
      constants `0' and `~0U' always work and they seem to be more readable.
   o  LSFLAG_SIGHANDLER gave its way to L_SIGHANDLER.
   o  The (unused) user flags field was replaced by a message type field;
      in the future, the streams will be able to filter the message based
      on a type.
   o  The `internal flags' field was renamed to `control bits'.