[sheepdog] decouple logger from dog

[MORITA Kazutaka]

At Tue, 12 Nov 2013 16:14:20 +0800,
Liu Yuan wrote:
> 
> Use fprintf directly as we need

What's the benefit?  sd logger is designed to be able to use fprintf()
for foreground programs.  In addition, we cannot completely decouple
logger from dog anyway because dog can call sd_emerg() in panic().

Thanks,

Kazutaka