Partial MFC of r212370. np@ requested this functionality so I kept in
the cxgb change, but otherwise MFC'd only the new function
sbuf_new_for_sysctl(9) and none of the changed code.

MFC r217830:

Document sbuf_new_for_sysctl(9).

Partial MFC of r217916:

Explicitly wire the user buffer rather than doing it implicitly in
sbuf_new_for_sysctl(9). This allows using an sbuf with a SYSCTL_OUT
drain for extremely large amounts of data where the caller knows that
appropriate references are held, and sleeping is not an issue.