add a print function for each log level

This adds the following functions.

   sd_emerg()  to log message with SDOG_EMERG
   sd_alert()  to log message with SDOG_ALERT
   sd_crit()   to log message with SDOG_CRIT
   sd_err()    to log message with SDOG_ERR
               (sd_eprintf is removed)
   sd_warn()   to log message with SDOG_WARN
   sd_notice() to log message with SDOG_NOTICE
   sd_info()   to log message with SDOG_INFO
               (sd_iprintf is removed)
   sd_debug()  to log message with SDOG_DEBUG
               (sd_dprintf is removed)

[fix sheep/sheep_priv.h - Yuan]

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
use sd_err instead of fprintf(stderr)

If logger is not initialized, sd_err works similarly to
fprintf(stderr).

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
logger: don't call log_write in sd_debug if possible

The aim of this patch is to optimize codes for normal log level and
not to hesitate to add a debug code even in a critical path.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>