Move optional package inclusions to the top, to make them easier to find.

Code elsewhere in this file tests for the inclusion of the package and does
not need to be uncommented.  Comments explain what they do and how to
disable them.

Added support for the optional "fancyhdr" package.  If used, page footers
include chapter information on the left and section information on the right.

Both optional packages, "fancyhdr" and "times", are enabled by default.  We
may want to disable them before shipping, but I'm not completely convinced.
(How many installations format their own documentation?)