Logging: Introduce inheritance of log_stream's.

Logging: Introduce inheritance of log_stream's.

The outbreak of various private fields (idata, pdata, udata) in struct log_stream
has gotten somewhat ugly, so I have replaced them by real private data allocated
after the standard portion of the log_stream structure.

log_new_stream() has gained a parameter telling it the size of the structure
to allocate. I have kept the recycler for the time being -- while saving allocations
is not worth the effort, it still nicely keeps the list of free stream ID's.

As idata are gone, the list of free items is now chained by log_stream->levels.

The default log stream no longer stores the fd explicitly, both log-file and
log-syslog keep private data in their own data structure extending log_stream.