Also make the various "set_state" functions naming consistent.
I also noticed that I needed to update the return values for
'nis.started'. The various <ds>.started functions should
return True if it's healthy, False if it's disabled, and raise
an exception if it's enabled and not healthy.

ypwhich has some strange and not very helpful failure modes. It
may return 1 indicating failure, or it may fail and return 0. Checking
for stderr is probably less problematic. If it returns 1, there is
also output on stderr so we can reduce the checks here.