sbuf(9): clarify kernel-only APIs

• move sbuf_bcopyin(9) and sbuf_copyin(9) near sbuf_new_for_sysctl(9), as all three functions are kernel-only APIs.
• add #ifdef _KERNEL around sbuf_*copyin and sbuf_new_for_sysctl(9) to make it visually clear that they are kernel-only APIs.

MFC after: 2 months
Sponsored by: Dell EMC Isilon