logger: clean printf styled log functions

Current sheep employes names "dprintf" and "vprintf" for its internal
logging usage. Unfortunately glibc provides functions which have same
names.

This patch clean these old functions and prepare new functions as
alternatives. New functions are:

sd_printf(): simple wrapper for log_write(). Caller has to pass log
	     level as its first parameter.
sd_dprintf(): call log_write() with SDOG_DEBUG.
sd_eprintf(): call log_write() with SDOG_ERROR.
sd_iprintf(): call log_write() with SDOG_INFO.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>