etcupdate and postinstall: make "-s src" compulsory

etcupdate and postinstall both behave, by default, as if
"-s /usr/src"
was specified.  I believe that this is not useful, because:

  * many people keep their sources elsewhere than /usr/src;

  * etcupdate, when given a source directory, attempts to
run "make
    distribution" in $/etc, and this often
fails.  Fixing the
    make failures is non trivial, and I really don't care
about fixing
    them because I always recommend that people should use
"etcupdate -s
    etc.tgz".

So, I'd like to make it an error to run postinstall or
etcupdate without
giving a "-s" arg.  People can still use "-s
/usr/src" to get the
existing (IMHO broken) behaviour.

--apb (Alan Barrett)