Make good on my promise to finally clean up the config clobbering.

Make good on my promise to finally clean up the config clobbering.
If you invoke config with the `-n' flag or have NO_CONFIG_CLOBBER in
your environment, config will behave the same way it used to.  This is
now _documented_ as well.  Rip out all the CONFIG_DONT_CLOBBER cruft;
some of it wasn't even correct anyway.