Don't allocate memory for operations that do not insert

Almost every operation performed on an nvlist was allocating a
new string to hold the key name. The nvlist_exists* family of
functions would always return false if they failed to allocate
the string. The rest of the functions would outright abort().
Fix the non-varargs variants of the functions to perform the
requested operations directly and the varargs versions to
allocate the string and call into the non-varargs versions.
The varargs versions are still broken and really can't be fixed,
so we might consider axing them entirely. However, now the non-
varargs functions are always safe to call.

Differential Revision: https://reviews.freebsd.org/D1879
Reviewed by: pjd, jfv
MFC after: 1 month
Sponsored by: Sandvine Inc.