Make update.sh accept command line options that are passed to CVSup.

Now users can schedule CVSup e.g. as follows:

45 0-6,8-23 * * * root nice -15 /usr/local/etc/cvsup/update.sh -s
45 7        * * * root nice -15 /usr/local/etc/cvsup/update.sh

Approved by: John Polstra <jdp@polstra.com>