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 ${SRCDIR}/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)