Expand sbufs to be able to output directly to the log+console interface

[scottl]

Summary

Refactor putbuf() so that it can be used as a generic output sink. Add
sbuf_putbuf() to utilize this. This allows buffers to be written
atomically without bouncing through the buffer length limitations of
printf() and PRINTF_BUFR_SIZE.

[imp]

Looks reasonable, assuming the code cut and paste into prf_putbuf was correct to start with.

[ken]

I think this looks good, but we should also look at what it would take to put a drain mode in as well.

[scottl]

Update patch to provide a kernel version and userland version
of sbuf_putbuf()

[scottl]

Need input on naming. sbuf_putbuf() was named since it wraps putbuf(). Is this the best name, though? Also, should it be kernel only, and the userland be something else that suggests printing to stdout vs stderr vs some other file descriptor?

[imp]

I can't think of a better name....

[wblock]

In D9601#202724, @imp wrote:

I can't think of a better name....

dj_mc_sbuf_putbuff_iii_this_time_its_personal_esq is the obvious choice.

Please remember to bump .Dd, and credit me in the comments for the name.