Implement runtime logging config option to cnid dbd backend. See #2721956.

Implement runtime logging config option to cnid dbd backend. See #2721956.
New option: -l <logconfig>. cnid_metad passed that to cnid_dbd !
Fixed: 0 is a valid fd (in logger.c) !
Added logger API documentation in logger.h.

Modified Files:
    etc/afpd/afp_options.c
    etc/cnid_dbd/cnid_metad.c
    etc/cnid_dbd/main.c
    include/atalk/logger.h
    libatalk/util/logger.c
    libatalk/util/test/logger_test.c