OMakeroot: fix doc target

Was not working before. The -html is option is required and output
directory must exist.
OMakeroot: reorder definitions into meaningful sections
OMakeroot: support user overrides of install paths

Now each variable has a default value, which can be overridden by the
contents of a configure.om file, which can be further overridden on the
omake command line.
OMakeroot: lots more fixes