Added dynamic log-levels:

Added dynamic log-levels:
  - yaz_log_mask_str (and _x) can now be called with a previously unknown
  name, which will get a new log bit assigned. This happens in most programs
  when parsing the -v command line option.
  - yaz_log_module_level can be called with a name, and if that name has
  got a bit defined, this bit will be returned, otherwise zero. This
  can then be used as the log_level in yaz_log and logf calls.
  - the names are cleaned from prefix paths and suffixes, so ./foo.c becomes
  foo.
Also added a routine to set the maximum log size, and a few comments in log.h