user-manual: add section ID's

Any section lacking an id gets an annoying warning when you build
the manual.  More seriously, the table of contents then generates
volatile id's which change with every build, with the effect that
we get URL's that change all the time.

The ID's are manually generated and sometimes inconsistent, but
that's OK.

XXX: what to do about the preface?

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>