introduce logging api and convert all library output to use it

 - multiple debug context calls lwsl_ err, warn, debug, parser, ext, client

 - api added to set which contexts output to stderr using a bitfield log_level

 - --disable-debug on configure removes all code that is not err or warn severity

 - err and warn contexts always output to stderr unless disabled by log_level

 - err and warn enabled by default in log_level

Signed-off-by: Andy Green <andy@warmcat.com>