Don't allocate memory for operations that do not insert

[rstone]

Summary

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.

[pjd]

lib/libnv/nvlist.c

276 ↗

(On Diff #3822)

'false' should be returned here.

1094 ↗

(On Diff #3822)

'false' instead of '0'.

1111 ↗

(On Diff #3822)

'false', not '0'.

[rstone]

Closed by commit rS279435 (authored by @rstone).