Don't allocate memory for operations that do not insert

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.