Preliminary, no-op support for sets.

Also, the beginnings of a new testing infrastructure!
Foundation for nstringset, too.
Add some nstringset functions: addition and membership checking.

nstringset_add(), nstringset_add_cstr(), nstringset_include() and
nstringset_include_cstr() are now implemented.
Added a man page for nset.

Also updated the other man pages to reflect that sets now exist.