nv: Add <sys/_nv.h> header for nvlist_t declaration

[jhb]

Summary

This can be useful for headers that wish to use nvlist_t pointers in a
structure or function argument without pulling in all of the headers
from <sys/nv.h>.

Sponsored by: Chelsio Communications

[imp]

sys/sys/cnv.h

40

Can't this be removed, here and elsewhere, since _nv.h includes it for !KERNEL and this is in !KERNEL?

[jhb]

One thing I did not try to address is the other places that define nvlist_t (notably in libcasper). They do so _after_ including <sys/nv.h> which is kind of confusing to me and I'm not sure why that is so I've left them alone.

sys/sys/cnv.h

40

Yes in theory. The reason I did not is that conceptually I just want <sys/_nv.h> to ensure that nvlist_t is defined. These headers define other symbols that <sys/nv_namespace.h> renames such as cnvlist_*, etc.

[imp]

Ok. Those are good reasons.