fix spelling error in radvd.conf.5.man
drop singleprocess option

Between 1.8.3 and 1.8.4 (Jan 2, 2012), the option to run radvd as a single
process was deprecated.  Radvd always drops privileges and forks a process
to do privileged operations.

Now that we're about to release version 1.10.0, it's time to remove the
rest of the option.