compat nvpair.h: make sure that the names are mangled only for kernel

Currently there is no good reason to mangle the userland API.
The change was introduced in eac1d566b46edef765754203bef22c75c1699966,
r279437. Also see https://reviews.freebsd.org/D1881.

I am still convinced that nv should not have introduced intentionally
conflicting API.

Discussed with: rstone
X-MFC with: r279437
Sponsored by: ClusterHQ