Adapt for userland compilation

off by 1

need sys/kmem.h for kmem_free()

PR kern/54406: the result of vasprintf (in the kernel) should be freed
by kmem_free(). Spotted by Mindaugas.

- nvpair_create_stringf: use the in-kernel vasprintf().
- Make nvlist_add_stringf() availabe in the kernel.

nvpair_remove_nvlist_array: revert part of the rev 1.4 change (it was applied
by mistake because the libnv upsteam code on Github has deviated from FreeBSD,
i.e. it has a different nvlist_set_array_n

nvpair_remove_nvlist_array: revert part of the rev 1.4 change (it was applied
by mistake because the libnv upsteam code on Github has deviated from FreeBSD,
i.e. it has a different nvlist_set_array_next() logic).

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libnv: Free the data array for NV_TYPE_DESCRIPTOR_ARRAY case.
Obtained from FreeBSD rev 343987 by oshogbo@.

libnv: fix multiple memory leaks.

- nvpair_create_stringv: free the temporary string; this fix affects
nvlist_add_stringf() and nvlist_add_stringv().

- nvpair_remove_nvlist_array (NV_TYPE_NVLIST_

libnv: fix multiple memory leaks.

- nvpair_create_stringv: free the temporary string; this fix affects
nvlist_add_stringf() and nvlist_add_stringv().

- nvpair_remove_nvlist_array (NV_TYPE_NVLIST_ARRAY case): free the chain
of nvpairs (as resetting it prevents nvlist_destroy() from freeing it).
Note: freeing the chain in nvlist_destroy() is not sufficient, because
it would still leak through nvlist_take_nvlist_array(). This affects
all nvlist_*_nvlist_array() users.

Found by clang/gcc ASAN. These fixes have been contributed to the
upstream (FreeBSD) repository.

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fix kernel build.

Add NetBSD stuff.

Import libnv from FreeBSD