Add function to force an nvlist into the error state

[rstone]

Summary

Add an nvlist_set_error() function that can be used to force an
nvlist into the error state. This is useful both for writing
tests and for writing APIs that use nvlists internally.

[pjd]

Other that the minor nit above, it looks good.

lib/libnv/nvlist.c

144

I'd assert that error is != 0.